Coding pattern with flags for determining tag data or block data

ABSTRACT

A substrate having a coding pattern disposed on a surface thereof. The coding pattern comprises a plurality of contiguous tags. Each tag comprises: a plurality of data symbols, including a plurality of first data symbols; a plurality of data elements; and one or more flags. The flags indicated either that: the first data symbols define first data contained in the tag; or the first data symbols define a fragment of second data. The second data is embedded in a block of said tags and is typically a public-key digital signature. The first data is typically a secret-key digital signature.

FIELD OF INVENTION

The present invention relates to a position-coding pattern on a surface.

COPENDING APPLICATIONS

The following applications have been filed by the Applicantsimultaneously with the present application:

NPT065US NPT066US NPT067US NPT068US NPT069US NPT070US NPT071US NPT072USNPT073US NPT074US NPT075US NPT076US NPT077US NPT078US NPT079US NPT081USNPT082US

The disclosures of these co-pending applications are incorporated hereinby reference. The above applications have been identified by theirfiling docket number, which will be substituted with the correspondingapplication number, once assigned.

CROSS REFERENCES

The following patents or patent applications filed by the applicant orassignee of the present invention are hereby incorporated bycross-reference.

10/815,621 10/815,635 10/815,647 11/488,162 10/815,636 11/041,65211/041,609 11/041,556 10/815,609 7,204,941 7,278,727 10/913,3807,122,076 7,156,289 09/575,197 6,720,985 7,295,839 09/722,174 7,068,3827,094,910 7,062,651 6,644,642 6,549,935 6,987,573 6,727,996 6,760,1197,064,851 6,290,349 6,428,155 6,785,016 6,831,682 6,741,871 6,965,43910/932,044 6,870,966 6,474,888 6,724,374 6,788,982 7,263,270 6,788,2936,737,591 09/693,514 10/778,056 10/778,061 11/193,482 7,055,7396,830,196 7,182,247 7,082,562 10/409,864 7,108,192 10/492,169 10/492,15210/492,168 10/492,161 7,308,148 6,957,768 7,170,499 11,856,06111/672,522 11/672,950 11,754,310 12,015,507 7,148,345

BACKGROUND

The Applicant has previously described a method of enabling users toaccess information from a computer system via a printed substrate e.g.paper. The substrate has a coding pattern printed thereon, which is readby an optical sensing device when the user interacts with with thesubstrate using the sensing device. A computer receives interaction datafrom the sensing device and uses this data to determine what action isbeing requested by the user. For example, a user may make handwritteninput onto a form or make a selection gesture around a printed item.This input is interpreted by the computer system with reference to apage description corresponding to the printed substrate.

It would desirable to improve the coding pattern on the substrate so asto maximize usage of images captured by the sensing device.

SUMMARY OF INVENTION

In a first aspect the present invention provides a substrate having acoding pattern disposed on a surface thereof, said coding patterncomprising:

-   -   a plurality of contiguous square tags of length l, each tag        comprising x-coordinate data and y-coordinate data, wherein a        y-axis is nominally defined as north-south and an x-axis is        nominally defined as east-west; and    -   a plurality of data elements contained in each tag, said        x-coordinate data being represented by a respective set of data        elements and said y-coordinate data being represented by a        respective set of data elements,        wherein:    -   said x-coordinate data comprises a replicated part and a        non-replicated part, said replicated part having a first        replication in a western half of said tag and a second        replication in an eastern half of said tag, and said        non-replicated part being represented in a central column of        said tag, said central column dividing said western half from        said eastern half;    -   said y-coordinate data comprises a replicated part and a        non-replicated part, said replicated part having a first        replication in a northern half of said tag and a second        replication in a southern half of said tag, and said        non-replicated part being represented in a central row of said        tag, said central row dividing said northern half from said        southern half; and    -   said central column and said central row each have a width q,        such that any square portion of said coding pattern having a        length (l+q) is guaranteed to contain said x-coordinate data and        said y-coordinate data for a tag irrespective of whether a whole        tag is contained in said portion.

Optionally, said coding pattern comprises:

-   -   a plurality of target elements defining a target grid, said        target grid comprising a plurality of cells, wherein neighboring        cells share target elements and wherein each tag is defined by a        plurality of contiguous cells.

Optionally, each tag comprises M² contiguous square cells, wherein M isan integer having a value of at least 1.

Optionally, said data elements are macrodots.

Optionally, q=2s, and s is defined as a spacing between adjacentmacrodots.

Optionally, a portion of data is represented by a macrodot occupying oneof a plurality of possible positions within a cell, each positionrepresenting one of a plurality of possible data values.

Optionally, a n-bit portion of data is represented by a macrodotoccupying one of 2^(n) possible positions within a cell, each positionrepresenting one of 2^(n) possible data values, wherein n is an integer.

Optionally, each cell defines a symbol group, each symbol groupcomprising a plurality of Reed-Solomon symbols encoded by a plurality ofsaid data elements.

Optionally, said x-coordinate data is encoded as an x-coordinatecodeword comprised of a respective set of Reed-Solomon symbols, and saidy-coordinate data is encoded as a y-coordinate codeword comprised of arespective set of Reed-Solomon symbols, and wherein at least somex-coordinate symbols occupy said central column and at least somey-coordinate symbols occupy said central row.

Optionally, each symbol comprises two square halves of length r, eachsquare half comprising 2 bits of data represented by a macrodotoccupying one of 4 possible positions within said half.

Optionally, r=2s and s is defined as a spacing between adjacentmacrodots.

Optionally, r=q.

Optionally, each tag comprises a plurality of common codewords, eachcommon codeword being comprised of a respective set of said Reed-Solomonsymbols, wherein said plurality of common codewords are defined ascodewords common to a plurality of contiguous tags.

Optionally, each symbol group comprises a fragment of at least one ofsaid common codewords, and contiguous symbol groups are arranged suchthat any tag-sized portion of said coding pattern is guaranteed tocontain said plurality of common codewords irrespective of whether awhole tag is contained in said portion.

Optionally, said one or more of said common codewords encode regionidentity data uniquely identifying a region of said surface.

Optionally, said region identity data uniquely identifies saidsubstrate.

Optionally, each cell comprises an orientation symbol encoded by atleast one data element, said orientation symbol identifying anorientation of said coding pattern with respect to said surface.

Optionally, each cell comprises one or more translation symbols encodedby a respective set of said data elements, said translation symbolsidentifying a translation of said cell relative to a tag containing saidcell.

Optionally, each cell comprises a pair of orthogonal translationsymbols, each orthogonal translation symbol identifying a respectiveorthogonal translation of said cell relative to a tag containing saidcell.

Optionally, said target elements are target dots and said data elementsare macrodots, and each target dot has a diameter of at least twice thatof each macrodot.

In a second aspect the present invention provides a substrate having acoding pattern disposed on a surface thereof, said coding patterncomprising a plurality of contiguous tags, each tag comprising:

-   -   a plurality of data symbols, which comprises a plurality of        first data symbols;    -   a plurality of data elements, each of said data symbols being        represented by a respective set of said data elements; and    -   one or more flags, said one or more flags indicating either        that:        -   said first data symbols define first data contained in said            tag; or        -   said first data symbols define a fragment of second data,            said second data being embedded in a block of said tags.

Optionally, said first data encodes a secret-key digital signature.

Optionally, said second data encodes a public-key digital signature.

Optionally, said first data symbols are arranged such that any tag-sizedportion of said coding pattern is guaranteed to contain said first datairrespective of whether a whole tag is contained in said portion.

Optionally, said surface comprises a plurality of blocks, such that saidsecond data can be assembled from a random or partial scan of saidsurface.

Optionally, said block has a width w of at least 2 tags and a height hof at least 2 tags.

Optionally, said one or more flags further indicate whether said tag iscontained in an active area of said surface.

Optionally, said active area is selected from the group comprising: ahyperlink area, a form field area and a button area.

Optionally, said coding pattern comprises:

-   -   a plurality of target elements defining a target grid, said        target grid comprising a plurality of cells, wherein neighboring        cells share target elements and wherein each tag is defined by a        plurality of contiguous cells.

Optionally, each tag comprises M² contiguous square cells, wherein M isan integer having a value of at least 2.

Optionally, a portion of data is represented by a macrodot occupying oneof a plurality of possible positions within a cell, each positionrepresenting one of a plurality of possible data values.

Optionally, a n-bit portion of data is represented by a macrodotoccupying one of 2^(n) possible positions within a cell, each positionrepresenting one of 2^(n) possible data values, wherein n is an integer.

Optionally, each cell defines a symbol group, each symbol groupcomprising a plurality of data symbols.

In a third aspect the present invention provides a method of imaging acoding pattern disposed on a surface of a substrate, said methodcomprising the steps of:

-   -   (a) operatively positioning an optical reader relative to said        surface and capturing an image of a portion of said coding        pattern, said coding pattern comprising:    -   a plurality of contiguous square tags of length l, each tag        comprising x-coordinate data and y-coordinate data, wherein a        y-axis is nominally defined as north-south and an x-axis is        nominally defined as east-west; and    -   a plurality of data elements contained in each tag, said        x-coordinate data being represented by a respective set of data        elements and said y-coordinate data being represented by a        respective set of data elements, wherein:    -   said x-coordinate data comprises a replicated part and a        non-replicated part, said replicated part having a first        replication in a western half of said tag and a second        replication in an eastern half of said tag, and said        non-replicated part being represented in a central column of        said tag, said central column dividing said western half from        said eastern half;    -   said y-coordinate data comprises a replicated part and a        non-replicated part, said replicated part having a first        replication in a northern half of said tag and a second        replication in a southern half of said tag, and said        non-replicated part being represented in a central row of said        tag, said central row dividing said northern half from said        southern half; and    -   said central column and said central row each have a width q;    -   (b) sampling and decoding x-coordinate data and y-coordinate        data within said imaged portion; and    -   (c) determining a position of said pen,

wherein said portion has a diameter of at least (l+q)√2 and less than(2l)√2.

Optionally, said coding pattern comprises:

-   -   a plurality of target elements defining a target grid, said        target grid comprising a plurality of cells, wherein neighboring        cells share target elements and wherein each tag is defined by a        plurality of contiguous cells.

Optionally, each tag comprises M² contiguous square cells, wherein M isan integer having a value of at least 1.

Optionally, said data elements are macrodots.

Optionally, q=2s, and s is defined as a spacing between adjacentmacrodots.

Optionally, a portion of data is represented by a macrodot occupying oneof a plurality of possible positions within a cell, each positionrepresenting one of a plurality of possible data values.

Optionally, each cell defines a symbol group, each symbol groupcomprising a plurality of Reed-Solomon symbols encoded by a plurality ofsaid data elements.

Optionally, said x-coordinate data is encoded as an x-coordinatecodeword comprised of a respective set of Reed-Solomon symbols, and saidy-coordinate data is encoded as a y-coordinate codeword comprised of arespective set of Reed-Solomon symbols, and wherein at least somex-coordinate symbols occupy said central column and at least somey-coordinate symbols occupy said central row.

Optionally, each symbol comprises two square halves of length r, eachsquare half comprising 2 bits of data represented by a macrodotoccupying one of 4 possible positions within said half.

Optionally, q=r=2s and s is defined as a spacing between adjacentmacrodots.

In a further aspect there is provided a system for imaging a codingpattern disposed on a surface of a substrate, said system comprising:

-   (A) said substrate, wherein said coding pattern comprises:    -   a plurality of contiguous square tags of length l, each tag        comprising x-coordinate data and y-coordinate data, wherein a        y-axis is nominally defined as north-south and an x-axis is        nominally defined as east-west; and    -   a plurality of data elements contained in each tag, said        x-coordinate data being represented by a respective set of data        elements and said y-coordinate data being represented by a        respective set of data elements, wherein:    -   said x-coordinate data comprises a replicated part and a        non-replicated part, said replicated part having a first        replication in a western half of said tag and a second        replication in an eastern half of said tag, and said        non-replicated part being represented in a central column of        said tag, said central column dividing said western half from        said eastern half;    -   said y-coordinate data comprises a replicated part and a        non-replicated part, said replicated part having a first        replication in a northern half of said tag and a second        replication in a southern half of said tag, and said        non-replicated part being represented in a central row of said        tag, said central row dividing said northern half from said        southern half; and    -   said central column and said central row each have a width q;        and-   (B) an optical reader comprising:    -   an image sensor for capturing an image of a portion of said        coding pattern, said image sensor having a field-of-view of at        least (l+q)√2 and less than (2l)√2; and    -   a processor configured for performing the steps of:        -   (i) sampling and decoding x-coordinate data and y-coordinate            data contained in an imaged portion; and        -   (ii) determining a position of said pen.

Optionally, said coding pattern comprises:

-   -   a plurality of target elements defining a target grid, said        target grid comprising a plurality of cells, wherein neighboring        cells share target elements and wherein each tag is defined by a        plurality of contiguous cells.

Optionally, each tag comprises M² contiguous square cells, wherein M isan integer having a value of at least 1.

Optionally, said data elements are macrodots, s is defined as a spacingbetween adjacent macrodots, and q=2s.

Optionally, a portion of data is represented by a macrodot occupying oneof a plurality of possible positions within a cell, each positionrepresenting one of a plurality of possible data values.

Optionally, each cell defines a symbol group, each symbol groupcomprising a plurality of Reed-Solomon symbols encoded by a plurality ofsaid data elements.

Optionally, said x-coordinate data is encoded as an x-coordinatecodeword comprised of a respective set of Reed-Solomon symbols, and saidy-coordinate data is encoded as a y-coordinate codeword comprised of arespective set of Reed-Solomon symbols, and wherein at least somex-coordinate symbols occupy said central column and at least somey-coordinate symbols occupy said central row.

Optionally, each symbol comprises two square halves of length r, eachsquare half comprising 2 bits of data represented by a macrodotoccupying one of 4 possible positions within said half.

Optionally, q=r=2s and s is defined as a spacing between adjacentmacrodots.

Optionally, said reader is an optically imaging pen having a nib.

Optionally, each data symbol comprises two halves, each half comprising2 bits of data represented by a macrodot occupying one of 4 possiblepositions within said half.

Optionally, each tag comprises a plurality of second data symbolsdefining at least one local codeword contained in said tag, said atleast one local codeword identifying a location of a respective tag.

Optionally, each tag comprises a plurality of third data symbols, saidthird data symbols defining one or more common codewords contained insaid tag, wherein said one or more common codewords are defined ascodewords common to a plurality of contiguous tags.

Optionally, said third data symbols are arranged such that any tag-sizedportion of said coding pattern is guaranteed to contain said one or morecommon codewords irrespective of whether a whole tag is contained insaid portion.

Optionally, said one or more common codewords encode region identitydata uniquely identifying a region of said surface.

Optionally, each cell comprises an orientation symbol encoded by atleast one data element, said orientation symbol identifying anorientation of said coding pattern with respect to said surface.

Optionally, each cell comprises one or more translation symbols encodedby a respective set of said data elements, said translation symbolsidentifying a translation of said cell relative to a tag containing saidcell.

In a fourth aspect the present invention provides a method of imaging acoding pattern disposed on a surface of a substrate, said methodcomprising the steps of:

-   -   (a) operatively positioning an optical reader relative to said        surface and capturing an image of a portion of said coding        pattern, said coding pattern comprising a plurality of        contiguous tags, each tag comprising:        -   a plurality of data symbols, which comprises a plurality of            first data symbols;        -   a plurality of data elements, each of said data symbols            being represented by a respective set of said data elements;            and        -   one or more flags, said one or more flags indicating either            that:            -   said first data symbols define a first data contained in                said tag; or            -   said first data symbols define a fragment of second                data, said second data being embedded in a block of said                tags.    -   (b) sampling and decoding said one or more flags;    -   (c) determining whether said first data symbols define said        first data or a fragment of said second data;    -   (d) sampling said first data symbols; and    -   (e) (1) decoding said first data if it is determined that said        first data symbols define said first data; or otherwise        -   (2) storing said sampled first data symbols in a memory of            said reader and assembling stored first data symbols into            said second data when sufficient first data symbols have            been sampled;

wherein said portion has a diameter of less than a block of tags.

Optionally, said first data encodes a secret-key digital signature.

Optionally, said second data encodes a public-key digital signature.

Optionally, said first data symbols are arranged such that any tag-sizedportion of said coding pattern is guaranteed to contain said first datairrespective of whether a whole tag is contained in said portion.

Optionally, said block has a width w of at least 2 tags and a height hof at least 2 tags.

Optionally, said surface comprises a plurality of blocks.

Optionally, said first data symbols define a fragment of said seconddata, and said method comprises the step of:

-   -   reporting to the user that further data sampling is required to        acquire said second data.

Optionally, said first data symbols define a fragment of said seconddata, and said method comprises the step of:

-   -   reporting to the user when sufficient fragments have been        retrieved.

Optionally, said one or more flags further indicate whether said tag iscontained in an active area of said surface.

Optionally, said active area is selected from the group comprising: ahyperlink area, a form field area and a button area.

Optionally, the method comprising the step of:

-   -   reporting to the user when said reader is positioned in an        active area.

In a further aspect there is provided a system for imaging a codingpattern disposed on a surface of a substrate, said system comprising:

-   (A) said substrate, wherein said coding pattern comprises a    plurality of contiguous tags, each tag comprising:    -   a plurality of data symbols, which comprises a plurality of        first data symbols;    -   a plurality of data elements, each of said data symbols being        represented by a respective set of said data elements; and    -   one or more flags, said one or more flags indicating either        that:        -   said first data symbols define first data contained in said            tag; or        -   said first data symbols define a fragment of second data,            said second data being embedded in a block of said tags.-   (B) an optical reader comprising:    -   an image sensor for capturing an image of a portion of said        coding pattern, said image sensor having a field-of-view of less        than a block of tags; and    -   a processor configured for performing the steps of:        -   (i) sampling and decoding said one or more flags;        -   (ii) determining whether said first data symbols define said            first data or a fragment of said second data;        -   (iii) sampling said first data symbols; and        -   (iv) (1) decoding said first data if it is determined that            said first data symbols define said first data; or otherwise            -   (2) storing said sampled first data symbols in a memory                of said reader and assembling stored first data symbols                into said second data when sufficient first data symbols                have been sampled.

Optionally, said first data encodes a secret-key digital signature.

Optionally, said second data encodes a public-key digital signature.

Optionally, said surface comprises a plurality of blocks.

Optionally, said reader comprises at least one of:

-   -   means for reporting to a user that said coding pattern contains        said second data;    -   means for reporting to a user that further data sampling is        required to acquire said second data; and    -   means for reporting to a user when sufficient fragments of said        second data have been retrieved.

Optionally, said one or more flags further indicate whether said tag iscontained in an active area of said surface.

Optionally, said active area is selected from the group comprising: ahyperlink area, a form field area and a button area.

Optionally, said reader comprises means for reporting to the user whensaid reader is positioned in an active area.

Optionally, said reader is an optically imaging pen having a nib.

In a sixth aspect the present invention provides a substrate having acoding pattern disposed on a surface thereof, said coding patterncomprising:

-   -   a plurality of target elements defining a target grid, said        target grid comprising a plurality of cells, wherein neighboring        cells share target elements;    -   a plurality of data elements contained in each cell; and    -   a plurality of tags, each tag being defined by a plurality of        contiguous cells, each tag comprising respective local tag data        encoded by a respective set of said data elements,

-   wherein said data elements encode data values by pulse position    modulation.

Optionally, said data elements are macrodots.

Optionally, a portion of data is represented by a macrodot occupying oneof a plurality of possible positions within a cell, each positionrepresenting one of a plurality of possible data values.

Optionally, a n-bit portion of data is represented by a macrodotoccupying one of 2^(n) possible positions within a cell, each positionrepresenting one of 2^(n) possible data values, wherein n is an integer.

Optionally, each cell defines a symbol group, each symbol groupcomprising a plurality of Reed-Solomon symbols encoded by a plurality ofsaid data elements.

Optionally, each symbol comprises two halves, each half comprising 2bits of data represented by a macrodot occupying one of 4 possiblepositions within said half.

Optionally, said local tag data is encoded as a local codeword comprisedof a set of said Reed-Solomon symbols.

Optionally, each tag comprises a plurality of replications of said localcodeword, such that any tag-sized portion of said coding pattern isguaranteed to contain said local codeword irrespective of whether awhole tag is contained in said portion.

Optionally, each tag is square and comprises four replications of saidlocal codeword, each replication being positioned within a respectivequarter of said tag.

Optionally, each local codeword identifies a location of a respectivetag.

Optionally, each tag comprises one or more common codewords, each commoncodeword being comprised of a set of said Reed-Solomon symbols, whereinsaid one or more common codewords are defined as codewords common to aplurality of contiguous tags.

Optionally, each symbol group comprises a fragment of at least one ofsaid one or more common codewords, and contiguous symbol groups arearranged such that any tag-sized portion of said coding pattern isguaranteed to contain said one or more common codewords irrespective ofwhether a whole tag is contained in said portion.

Optionally, said one or more common codewords encode region identitydata uniquely identifying a region of said surface.

Optionally, said one or more common codewords uniquely identifies saidsubstrate. Optionally, each cell comprises an orientation symbol encodedby at least one data element, said orientation symbol identifying anorientation of said coding pattern with respect to said surface.

Optionally, each cell comprises one or more translation symbols encodedby a respective set of said data elements, said translation symbolsidentifying a translation of said cell relative to a tag containing saidcell.

Optionally, each cell comprises a pair of orthogonal translationsymbols, each orthogonal translation symbol identifying a respectiveorthogonal translation of said cell relative to a tag containing saidcell.

Optionally, each tag is square and comprises M² contiguous square cells,wherein M is an integer having a value of at least 2.

Optionally, said target elements are sufficiently large to bedistinguishable from said data elements by a low-pass filter.

Optionally, said target elements are target dots and said data elementsare macrodots, and wherein each target dot has a diameter of at leasttwice that of each macrodot.

In a fifth aspect the present invention provides a substrate having acoding pattern disposed on a surface thereof, said coding patterncomprising:

-   -   a plurality of target elements defining a target grid, said        target grid comprising a plurality of cells, wherein neighboring        cells share target elements;    -   a plurality of data elements contained in each cell; and    -   a plurality of tags, each tag being defined by a plurality of        contiguous cells, each tag comprising respective local tag data        encoded by a respective set of said data elements,

-   wherein each tag comprises at least 9 target elements.

Optionally, each tag comprises at least 16 target elements.

Optionally, each tag comprises at least 25 target elements.

Optionally, each tag is square and comprises M² contiguous square cells,wherein M is an integer having a value of at least 2.

Optionally, said target elements are configured to facilitatecomputation of a perspective distortion of said target grid when aportion of said coding pattern is acquired by an optical sensing device.

Optionally, said target elements are sufficiently large to bedistinguishable from said data elements by a low-pass filter.

Optionally, said target elements are target dots and said data elementsare macrodots.

Optionally, each target dot has a diameter of at least twice that ofeach macrodot.

Optionally, said macrodots encode data values by pulse positionmodulation.

Optionally, a n-bit portion of data is represented by a macrodotoccupying one of 2^(n) possible positions within a cell, each positionrepresenting one of 2^(n) possible data values, wherein n is an integer.

Optionally, each tag comprises a plurality of replications of said localtag data, such that any tag-sized portion of said coding pattern isguaranteed to contain said local tag data irrespective of whether awhole tag is contained in said portion.

Optionally, each tag is square and comprises four replications of saidlocal tag data, each replication being positioned within a respectivequarter of said tag.

Optionally, said local tag data identifies a location of a respectivetag.

Optionally, each tag comprises common data encoded by a respective setof said data elements, wherein said common data is defined as datacommon to a plurality of contiguous tags.

Optionally, each cell comprises a fragment of said common data, andcontiguous cells are arranged such that any tag-sized portion of saidcoding pattern is guaranteed to contain said common data irrespective ofwhether a whole tag is contained in said portion.

Optionally, said common data is region identity data uniquelyidentifying a region of said surface.

Optionally, said common data uniquely identifies said substrate.

Optionally, each cell comprises orientation data encoded by a respectiveset of said data elements, said orientation data identifying anorientation of said coding pattern with respect to said surface.

Optionally, each cell comprises translation data encoded by a respectiveset of said data elements, said translation data identifying atranslation of said cell relative to a tag containing said cell.

Optionally, each cell defines a symbol group, each symbol groupcomprising a plurality of Reed-Solomon symbols encoded by a plurality ofsaid data elements.

In a seventh aspect the present invention provides a substrate having acoding pattern disposed on a surface thereof, said coding patterncomprising:

-   -   a plurality of target elements defining a target grid, said        target grid comprising a plurality of cells, wherein neighboring        cells share target elements;    -   a plurality of data elements contained in each cell; and    -   a plurality of tags, each tag being defined by a plurality of        contiguous cells, each tag comprising respective tag data        encoded by a respective set of said data elements,

-   wherein each cell comprises at least one orientation symbol encoded    by at least one data element, such that any tag-sized portion of    said coding pattern is guaranteed to contain a plurality of said    orientation symbols, each orientation symbol identifying an    orientation of a layout of said tag data with respect to said target    grid.

Optionally, each orientation symbol comprises a data element positionedat one of four possible positions within each cell, each positionrepresenting one of four possible orientations.

Optionally, each orientation symbol is readable by an optical sensingdevice at any of said four orientations.

-   Optionally, each tag comprises N cells, and at least N orientation    symbols form an orientation code with minimum distance N, wherein N    is an integer having a value of at least 4.

Optionally, said cells are arranged such that any tag-sized portion ofsaid coding pattern is guaranteed to contain said orientation codecomprising at least N orientation symbols.

Optionally, said data elements are macrodots.

Optionally, a portion of data is represented by a macrodot occupying oneof a plurality of possible positions within a cell, each positionrepresenting one of a plurality of possible data values.

Optionally, a n-bit portion of data is represented by a macrodotoccupying one of 2^(n) possible positions within a cell, each positionrepresenting one of 2^(n) possible data values, wherein n is an integer.

Optionally, each cell defines a symbol group, each symbol groupcomprising a plurality of Reed-Solomon symbols encoded by a plurality ofsaid data elements.

Optionally, each orientation symbol identifies an orientation of alayout of said Reed-Solomon symbols with respect to said target grid.

Optionally, said tag data is encoded as a local codeword comprised of aset of said Reed-Solomon symbols.

Optionally, each tag comprises a plurality of replications of said localcodeword, such that any tag-sized portion of said coding pattern isguaranteed to contain said local codeword irrespective of whether awhole tag is contained in said portion.

Optionally, each tag is square and comprises four replications of saidlocal codeword, each replication being positioned within a respectivequarter of said tag.

Optionally, each local codeword identifies a location of a respectivetag.

Optionally, each tag comprises one or more common codewords, each commoncodeword being comprised of a set of said Reed-Solomon symbols, whereinsaid one or more common codewords are defined as codewords common to aplurality of contiguous tags.

Optionally, each symbol group comprises a fragment of at least one ofsaid one or more common codewords, and contiguous symbol groups arearranged such that any tag-sized portion of said coding pattern isguaranteed to contain said one or more common codewords irrespective ofwhether a whole tag is contained in said portion.

Optionally, said one or more common codewords encode region identitydata uniquely identifying a region of said surface.

Optionally, said one or more common codewords uniquely identifies saidsubstrate.

Optionally, each cell comprises one or more translation symbols encodedby a respective set of said data elements, said translation symbolsidentifying a translation of said cell relative to a tag containing saidcell.

Optionally, each cell comprises a pair of orthogonal translationsymbols, each orthogonal translation symbol identifying a respectiveorthogonal translation of said cell relative to a tag containing saidcell.

In an eighth aspect the present invention provides a substrate having acoding pattern disposed on a surface thereof, said coding patterncomprising:

-   -   a plurality of target elements defining a target grid, said        target grid comprising a plurality of cells, wherein neighboring        cells share target elements;    -   a plurality of data elements contained in each cell; and    -   a plurality of tags, each tag being defined by a plurality of        contiguous cells, each tag comprising respective local tag data        encoded by a respective set of said data elements, each tag        comprising common data encoded by a respective set of said data        elements, said common data being defined as data common to a        plurality of contiguous tags,

-   wherein each cell comprises a fragment of said common data, and    contiguous cells are arranged such that any tag-sized portion of    said coding pattern is guaranteed to contain said common data    irrespective of whether a whole tag is contained in said portion.

Optionally, said common data is region identity data uniquelyidentifying a region of said surface.

Optionally, said common data uniquely identifies said substrate.

Optionally, said data elements are macrodots.

Optionally, a portion of data is represented by a macrodot occupying oneof a plurality of possible positions within a cell, each positionrepresenting one of a plurality of possible data values.

Optionally, a n-bit portion of data is represented by a macrodotoccupying one of 2^(n) possible positions within a cell, each positionrepresenting one of 2^(n) possible data values, wherein n is an integer.

Optionally, each cell defines a symbol group, each symbol groupcomprising a plurality of Reed-Solomon symbols encoded by a plurality ofsaid data elements.

Optionally, each symbol comprises two halves, each half comprising 2bits of data represented by a macrodot occupying one of 4 possiblepositions within said half.

Optionally, said common data is encoded as one or more common codewords,each common codeword being comprised of a set of said Reed-Solomonsymbols.

Optionally, each symbol group comprises a fragment of at least one ofsaid one or more common codewords, and contiguous symbol groups arearranged such that any tag-sized portion of said coding pattern isguaranteed to contain said one or more common codewords irrespective ofwhether a whole tag is contained in said portion.

Optionally, said local tag data is encoded as a local codeword comprisedof a set of said Reed-Solomon symbols.

Optionally, each tag comprises a plurality of replications of said localcodeword, such that any tag-sized portion of said coding pattern isguaranteed to contain said local codeword irrespective of whether awhole tag is contained in said portion.

Optionally, each tag is square and comprises four replications of saidlocal codeword, each replication being positioned within a respectivequarter of said tag.

Optionally, said local tag data identifies a location of a respectivetag.

Optionally, each cell comprises an orientation symbol encoded by atleast one data element, said orientation symbol identifying anorientation of said coding pattern with respect to said surface.

Optionally, each cell comprises one or more translation symbols encodedby a respective set of said data elements, said translation symbolsidentifying a translation of said cell relative to a tag containing saidcell.

Optionally, each cell comprises a pair of orthogonal translationsymbols, each orthogonal translation symbol identifying a respectiveorthogonal translation of said cell relative to a tag containing saidcell.

Optionally, each tag is square and comprises M² contiguous square cells,wherein M is an integer having a value of at least 2.

Optionally, said target elements are sufficiently large to bedistinguishable from said data elements by a low-pass filter.

Optionally, said target elements are target dots and said data elementsare macrodots, and wherein each target dot has a diameter of at leasttwice that of each macrodot.

In a ninth aspect the present invention provides substrate having acoding pattern disposed on a surface thereof, said coding patterncomprising:

-   -   a plurality of target elements defining a target grid, said        target grid comprising a plurality of cells, wherein neighboring        cells share target elements;    -   a plurality of data elements contained in each cell; and    -   a plurality of tags, each tag being defined by a plurality of        contiguous cells, each tag comprising respective tag data        encoded by a respective set of said data elements,

-   wherein each cell comprises one or more translation symbols encoded    by a respective set of said data elements, said one or more    translation symbols identifying a translation of said cell relative    to a tag containing said cell.

Optionally, each cell comprises a pair of orthogonal translationsymbols, each orthogonal translation symbol identifying a respectiveorthogonal translation of said cell relative to a tag containing saidcell.

Optionally, each tag is square and comprises M² contiguous square cells,wherein M is an integer having a value of at least 2.

Optionally, M translation symbols in a row of M cells define a cyclicposition code having minimum distance M, said code being defined by afirst codeword.

Optionally, M translation symbols in a column of M cells define a cyclicposition code having minimum distance M, said code being defined by asecond codeword.

Optionally, each tag comprises N cells, and at least N translationsymbols form a third codeword with minimum distance N, wherein N is aninteger having a value of at least 4.

Optionally, any tag-sized portion of said coding pattern is guaranteedto contain at least N translation symbols, thereby capturing said thirdcodeword.

Optionally, each cell comprises at least one orientation symbol encodedby at least one data element, such that any tag-sized portion of saidcoding pattern is guaranteed to contain a plurality of said orientationsymbols, each orientation symbol identifying an orientation of saidcoding pattern with respect to said surface.

Optionally, said data elements are macrodots.

Optionally, a portion of data is represented by a macrodot occupying oneof a plurality of possible positions within a cell, each positionrepresenting one of a plurality of possible data values.

Optionally, a n-bit portion of data is represented by a macrodotoccupying one of 2^(n) possible positions within a cell, each positionrepresenting one of 2^(n) possible data values, wherein n is an integer.

Optionally, each cell defines a symbol group, each symbol groupcomprising a plurality of Reed-Solomon symbols encoded by a plurality ofsaid data elements.

Optionally, each symbol comprises two halves, each half comprising 2bits of data represented by a macrodot occupying one of 4 possiblepositions within said half.

Optionally, said tag data is encoded as a local codeword comprised of aset of said Reed-Solomon symbols.

Optionally, each tag comprises a plurality of replications of said localcodeword, such that any tag-sized portion of said coding pattern isguaranteed to contain said local codeword irrespective of whether awhole tag is contained in said portion.

Optionally, each tag is square and comprises four replications of saidlocal codeword, each replication being positioned within a respectivequarter of said tag.

Optionally, each local codeword identifies a location of a respectivetag.

Optionally, each tag comprises one or more common codewords, each commoncodeword being comprised of a set of said Reed-Solomon symbols, whereinsaid one or more common codewords are defined as codewords common to aplurality of contiguous tags.

Optionally, each symbol group comprises a fragment of at least one ofsaid one or more common codewords, and contiguous symbol groups arearranged such that any tag-sized portion of said coding pattern isguaranteed to contain said one or more common codewords irrespective ofwhether a whole tag is contained in said portion.

Optionally, said one or more common codewords encode region identitydata uniquely identifying a region of said surface.

BRIEF DESCRIPTION OF DRAWINGS

Preferred and other embodiments of the invention will now be described,by way of non-limiting example only, with reference to the accompanyingdrawings, in which:

FIG. 1 is a schematic of a the relationship between a sample printednetpage and its online page description;

FIG. 2 shows an embodiment of basic netpage architecture with variousalternatives for the relay device;

FIG. 3 shows the structure of a tag;

FIG. 4 shows a group of ten data symbols and four targets;

FIG. 5 shows the layout of a square data half-symbol;

FIG. 6 shows the layout of rectangular data symbols;

FIG. 7 shows the spacing of macrodot positions;

FIG. 8 shows an orientation code symbol layout;

FIG. 9 shows a translation code symbol layout;

FIG. 10 shows the layout of a flag symbol;

FIG. 11 shows a replicated x-coordinate codeword X with the first copyshown shaded;

FIG. 12 shows a replicated y-coordinate codeword Y with the first copyshown shaded;

FIG. 13 shows common codewords B, C and D, with codeword B shown shaded;

FIG. 14 shows the layout of a complete tag;

FIG. 15 shows the layout of a Reed-Solomon codeword;

FIG. 16 is a flowchart of image processing;

FIG. 17 shows a nib and elevation of the pen held by a user;

FIG. 18 shows the pen held by a user at a typical incline to a writingsurface;

FIG. 19 is a lateral cross section through the pen;

FIG. 20A is a bottom and nib end partial perspective of the pen;

FIG. 20B is a bottom and nib end partial perspective with the fields ofillumination and field of view of the sensor window shown in dottedoutline;

FIG. 21 is a longitudinal cross section of the pen;

FIG. 22A is a partial longitudinal cross section of the nib and barrelmolding;

FIG. 22B is a partial longitudinal cross section of the IR LED's and thebarrel molding;

FIG. 23 is a ray trace of the pen optics adjacent a sketch of the inkcartridge;

FIG. 24 is a side elevation of the lens;

FIG. 25 is a side elevation of the nib and the field of view of theoptical sensor; and

FIG. 26 is a block diagram of the pen electronics.

DETAILED DESCRIPTION OF PREFERRED AND OTHER EMBODIMENTS

1.1 Netpage System Architecture

In a preferred embodiment, the invention is configured to work with thenetpage networked computer system, a detailed overview of which follows.It will be appreciated that not every implementation will necessarilyembody all or even most of the specific details and extensions discussedbelow in relation to the basic system. However, the system is describedin its most complete form to reduce the need for external reference whenattempting to understand the context in which the preferred embodimentsand aspects of the present invention operate.

In brief summary, the preferred form of the netpage system employs acomputer interface in the form of a mapped surface, that is, a physicalsurface which contains references to a map of the surface maintained ina computer system. The map references can be queried by an appropriatesensing device. Depending upon the specific implementation, the mapreferences may be encoded visibly or invisibly, and defined in such away that a local query on the mapped surface yields an unambiguous mapreference both within the map and among different maps. The computersystem can contain information about features on the mapped surface, andsuch information can be retrieved based on map references supplied by asensing device used with the mapped surface. The information thusretrieved can take the form of actions which are initiated by thecomputer system on behalf of the operator in response to the operator'sinteraction with the surface features.

In its preferred form, the netpage system relies on the production of,and human interaction with, netpages. These are pages of text, graphicsand images printed on ordinary paper, but which work like interactivewebpages. Information is encoded on each page using ink which issubstantially invisible to the unaided human eye. The ink, however, andthereby the coded data, can be sensed by an optically imaging sensingdevice and transmitted to the netpage system. The sensing device maytake the form of a clicker (for clicking on a specific position on asurface), a pointer having a stylus (for pointing or gesturing on asurface using pointer strokes), or a pen having a marking nib (formarking a surface with ink when pointing, gesturing or writing on thesurface). References herein to “pen” or “netpage pen” are provided byway of example only. It will, of course, be appreciated that the pen maytake the form of any of the sensing devices described above.

In one embodiment, active buttons and hyperlinks on each page can beclicked with the sensing device to request information from the networkor to signal preferences to a network server. In one embodiment, textwritten by hand on a netpage is automatically recognized and convertedto computer text in the netpage system, allowing forms to be filled in.In other embodiments, signatures recorded on a netpage are automaticallyverified, allowing e-commerce transactions to be securely authorized. Inother embodiments, text on a netpage may be clicked or gestured toinitiate a search based on keywords indicated by the user.

As illustrated in FIG. 1, a printed netpage 1 can represent ainteractive form which can be filled in by the user both physically, onthe printed page, and “electronically”, via communication between thepen and the netpage system. The example shows a “Request” formcontaining name and address fields and a submit button. The netpage 1consists of graphic data 2, printed using visible ink, and a surfacecoding pattern 3 superimposed with the graphic data. The surface codingpattern 3 comprises a collection of tags 4. One such tag 4 is shown inthe shaded region of FIG. 1, although it will be appreciated thatcontiguous tags 4, defined by the coding pattern 3, are densely tiledover the whole netpage 1.

The corresponding page description 5, stored on the netpage network,describes the individual elements of the netpage. In particular itdescribes the type and spatial extent (zone) of each interactive element(i.e. text field or button in the example), to allow the netpage systemto correctly interpret input via the netpage. The submit button 6, forexample, has a zone 7 which corresponds to the spatial extent of thecorresponding graphic 8.

As illustrated in FIG. 2, a netpage sensing device 400, such as the pendescribed in Section 3, works in conjunction with a netpage relay device601, which is an Internet-connected device for home, office or mobileuse. The pen 400 is wireless and communicates securely with the netpagerelay device 601 via a short-range radio link 9. In an alternativeembodiment, the netpage pen 400 utilises a wired connection, such as aUSB or other serial connection, to the relay device 601.

The relay device 601 performs the basic function of relaying interactiondata to a page server 10, which interprets the interaction data. Asshown in FIG. 2, the relay device 601 may, for example, take the form ofa personal computer 601 a, a netpage printer 601 b or some other relay601 c (e.g. personal computer or mobile phone incorporating a webbrowser).

The netpage printer 601 b is able to deliver, periodically or on demand,personalized newspapers, magazines, catalogs, brochures and otherpublications, all printed at high quality as interactive netpages.Unlike a personal computer, the netpage printer is an appliance whichcan be, for example, wall-mounted adjacent to an area where the morningnews is first consumed, such as in a user's kitchen, near a breakfasttable, or near the household's point of departure for the day. It alsocomes in tabletop, desktop, portable and miniature versions. Netpagesprinted on-demand at their point of consumption combine the ease-of-useof paper with the timeliness and interactivity of an interactive medium.

Alternatively, the netpage relay device 601 may be a portable device,such as a mobile phone or PDA, a laptop or desktop computer, or aninformation appliance connected to a shared display, such as a TV. Ifthe relay device 601 is not a netpage printer 601 b which printsnetpages digitally and on demand, the netpages may be printed bytraditional analog printing presses, using such techniques as offsetlithography, flexography, screen printing, relief printing androtogravure, as well as by digital printing presses, using techniquessuch as drop-on-demand inkjet, continuous inkjet, dye transfer, andlaser printing.

As shown in FIG. 2, the netpage sensing device 400 interacts with aportion of the tag pattern on a printed netpage 1, or other printedsubstrate such as a label of a product item 251, and communicates, via ashort-range radio link 9, the interaction to the relay device 601. Therelay 601 sends corresponding interaction data to the relevant netpagepage server 10 for interpretation. Raw data received from the sensingdevice 400 may be relayed directly to the page server 10 as interactiondata. Alternatively, the interaction data may be encoded in the form ofan interaction URI and transmitted to the page server 10 via a user'sweb browser 601 c. The web browser 601 c may then receive a URI from thepage server 10 and access a webpage via a webserver 201. In somecircumstances, the page server 10 may access application computersoftware running on a netpage application server 13.

The netpage relay device 601 can be configured to support any number ofsensing devices, and a sensing device can work with any number ofnetpage relays. In the preferred implementation, each netpage sensingdevice 400 has a unique identifier. This allows each user to maintain adistinct profile with respect to a netpage page server 10 or applicationserver 13.

Digital, on-demand delivery of netpages 1 may be performed by thenetpage printer 601 b, which exploits the growing availability ofbroadband Internet access. Netpage publication servers 14 on the netpagenetwork are configured to deliver print-quality publications to netpageprinters. Periodical publications are delivered automatically tosubscribing netpage printers via pointcasting and multicasting Internetprotocols. Personalized publications are filtered and formattedaccording to individual user profiles.

A netpage pen may be registered with a netpage registration server 11and linked to one or more payment card accounts. This allows e-commercepayments to be securely authorized using the netpage pen. The netpageregistration server compares the signature captured by the netpage penwith a previously registered signature, allowing it to authenticate theuser's identity to an e-commerce server. Other biometrics can also beused to verify identity. One version of the netpage pen includesfingerprint scanning, verified in a similar way by the netpageregistration server.

1.2 Netpages

Netpages are the foundation on which a netpage network is built. Theyprovide a paper-based user interface to published information andinteractive services.

As shown in FIG. 1, a netpage consists of a printed page (or othersurface region) invisibly tagged with references to an onlinedescription 5 of the page. The online page description 5 is maintainedpersistently by the netpage page server 10. The page descriptiondescribes the visible layout and content of the page, including text,graphics and images. It also describes the input elements on the page,including buttons, hyperlinks, and input fields. A netpage allowsmarkings made with a netpage pen on its surface to be simultaneouslycaptured and processed by the netpage system.

Multiple netpages (for example, those printed by analog printingpresses) can share the same page description. However, to allow inputthrough otherwise identical pages to be distinguished, each netpage maybe assigned a unique page identifier. This page ID has sufficientprecision to distinguish between a very large number of netpages.

Each reference to the page description 5 is repeatedly encoded in thenetpage pattern. Each tag (and/or a collection of contiguous tags)identifies the unique page on which it appears, and thereby indirectlyidentifies the page description 5. Each tag also identifies its ownposition on the page. Characteristics of the tags are described in moredetail below.

Tags are typically printed in infrared-absorptive ink on any substratewhich is infrared-reflective, such as ordinary paper, or in infraredfluorescing ink. Near-infrared wavelengths are invisible to the humaneye but are easily sensed by a solid-state image sensor with anappropriate filter.

A tag is sensed by a 2D area image sensor in the netpage sensing device,and the tag data is transmitted to the netpage system via the nearestnetpage relay device 601. The pen 400 is wireless and communicates withthe netpage relay device 601 via a short-range radio link. It isimportant that the pen recognize the page ID and position on everyinteraction with the page, since the interaction is stateless. Tags areerror-correctably encoded to make them partially tolerant to surfacedamage.

The netpage page server 10 maintains a unique page instance for eachunique printed netpage, allowing it to maintain a distinct set ofuser-supplied values for input fields in the page description 5 for eachprinted netpage 1.

2 Netpage Tags

2.1 Tag Data Content

Each tag 4 identifies an absolute location of that tag within a regionof a substrate.

Each interaction with a netpage should also provide a region identitytogether with the tag location. In a preferred embodiment, the region towhich a tag refers coincides with an entire page, and the region ID istherefore synonymous with the page ID of the page on which the tagappears. In other embodiments, the region to which a tag refers can bean arbitrary subregion of a a page or other surface. For example, it cancoincide with the zone of an interactive element, in which case theregion ID can directly identify the interactive element.

As described in the Applicant's previous applications (e.g. U.S. Pat.No. 6,832,717), the region identity may be encoded discretely in eachtag 4. As will be described in more detail below, the region identitymay be encoded by a plurality of contiguous tags in such a way thatevery interaction with the substrate still identifies the regionidentity, even if a whole tag is not in the field of view of the sensingdevice.

Each tag 4 should preferably identify an orientation of the tag relativeto the substrate on which the tag is printed. Orientation data read froma tag enables the rotation (yaw) of the pen 101 relative to thesubstrate to be determined

A tag 4 may also encode one or more flags which relate to the region asa whole or to an individual tag. One or more flag bits may, for example,signal a sensing device to provide feedback indicative of a functionassociated with the immediate area of the tag, without the sensingdevice having to refer to a description of the region. A netpage penmay, for example, illuminate an “active area” LED when in the zone of ahyperlink.

A tag 4 may also encode a digital signature or a fragment thereof. Tagsencoding (partial) digital signatures are useful in applications whereit is required to verify a product's authenticity. Such applications aredescribed in, for example, US Publication No. 2007/0108285, the contentsof which is herein incorporated by reference. The digital signature maybe encoded in such a way that it can be retrieved from every interactionwith the substrate. Alternatively, the digital signature may be encodedin such a way that it can be assembled from a random or partial scan ofthe substrate.

It will, of course, be appreciated that other types of information (e.g.tag size etc) may also be encoded into each tag or a plurality of tags,as will be explained in more detail below.

2.2 General Tag Structure

As described above in connection with FIG. 1, the netpage surface codinggenerally consists of a dense planar tiling of tags. In the presentinvention, each tag 4 is represented by a coding pattern which containstwo kinds of elements. Referring to FIGS. 3 and 4, the first kind ofelement is a target element. Target elements in the form of target dots301 allow a tag 4 to be located in an image of a coded surface, andallow the perspective distortion of the tag to be inferred. The secondkind of element is a data element in the form of a macrodot 302 (seeFIG. 7). Each macrodot 302 encodes a data value. As described in theApplicant's earlier disclosures (e.g. U.S. Pat. No. 6,832,717), thepresence or absence of a macrodot was be used to represent a binary bit.However, the tag structure of the present invention encodes a data valueusing pulse position modulation, which is described in more detail inSection 2.3.

The coding pattern 3 is represented on the surface in such a way as toallow it to be acquired by an optical imaging system, and in particularby an optical system with a narrowband response in the near-infrared.The pattern 3 is typically printed onto the surface using a narrowbandnear-infrared ink.

FIG. 3 shows the structure of a complete tag 4 with target elements 301shown. The tag 4 is square and contains sixteen target elements. Thosetarget elements 301 located at the edges and corners of the tag (twelvein total) are shared by adjacent tags and define the perimeter of thetag. In contrast with the Applicant's previous tag designs, the highnumber of target elements 301 advantageously facilitates accuratedetermination of a perspective distortion of the tag 4 when it is imagedby the sensing device 101. This improves the accuracy of tag sensingand, ultimately, position determination.

The tag 4 consists of a square array of nine symbol groups 303. Symbolgroups 303 are demarcated by the target elements 301 so that each symbolgroup is contained within a square defined by four target elements.Adjacent symbol groups 303 are contiguous and share targets.

Since the target elements 301 are all identical, they do not demarcateone tag from its adjacent tags. Viewed purely at the level of targetelements, only symbol groups 303, which define cells of a target grid,can be distinguished—the tags 4 themselves are indistinguishable byviewing only the target elements. Hence, tags 4 must be aligned with thetarget grid as part of tag decoding.

The tag 4 is designed to allow all tag data, with the exception of anembedded data object (see Section 2.8.3), to be recovered from animaging field of view no larger than the size of the tag (plus onemacrodot unit). This implies that any data unique to the tag 4 mustappear four times within the tag—i.e. once in each quadrant or quarter;any data unique to a column or row of tags must appear twice within thetag—i.e. once in each horizontal half or vertical half of the tagrespectively; and any data common to a set of tags needs to appear oncewithin the tag.

2.3 Symbol Groups

As shown in FIG. 4, each of the nine symbol groups 303 comprises tendata symbols 304, each data symbol being part of a codeword. Inaddition, each symbol group 303 comprises an orientation code (‘OR’) andone symbol from each of two orthogonal translation codes (‘HT’ and‘VT’). The orientation code allows the orientation of the tag in thefield of view to be determined. The two orthogonal translation codesallow the translation of tag(s) relative to the symbol groups 303 in thefield of view to be determined. In other words, the translation codesenable alignment of the ‘invisible’ tags with the target grid.

Each symbol group 304 contains two symbols from a flag code (F). Theflag code encodes the active area flag.

Each symbol 304 contains four bits of data. Generally, each symbol 304is divided into two halves, and each of these two halves {h₀, h₁} isencoded using two-bit pulse position modulation, i.e. using a singlemacrodot 302 in one of four positions {p₀₀, p₀₁, p₁₀, p₁₁} in the half.The half h₀ encodes the least-significant bits of the symbol; the halfh₁ encodes the most-significant bits.

The two halves of one data symbol 304 need not necessarily be contiguouswithin the symbol group. FIG. 3 shows four half data symbols centeredaround the orientation symbol ‘OR’. Half-symbols 304A and 304B form thetwo halves of one whole data symbol.

FIG. 5 shows the layout of a square data half-symbol. FIG. 6 shows thelayout of two rectangular data symbols (a vertical rectangle symbol anda horizontal rectangle symbol), each of which comprises twohalf-symbols.

2.4 Targets and Macrodots

The spacing of macrodots 302 in both dimensions, as shown in FIG. 7, isspecified by the parameter s. It has a nominal value of 95 μm, based on6 dots printed at a pitch of 1600 dots per inch.

Only macrodots 302 are part of the representation of a symbol 304 in thepattern. The outline of a symbol 304 is shown in, for example, FIGS. 3and 4 merely to elucidate more clearly the structure of a tag.

A macrodot 302 is nominally circular with a nominal diameter of (4/6)s.However, it is allowed to vary in size by ±15% according to thecapabilities of the device used to produce the pattern.

A target 301 is nominally circular with a nominal diameter of (12/6)s.However, it is allowed to vary in size by ±15% according to thecapabilities of the device used to produce the pattern.

Each tag 4 has a width of 40s and a length of 40s.

The macrodot spacing, and therefore the overall scale of the tagpattern, is allowed to vary by ±11% according to the capabilities of thedevice used to produce the pattern. Any deviation from the nominal scaleis recorded in each tag (in a tag size ID field) to allow accurategeneration of position samples.

These tolerances are independent of one another. They may be refinedwith reference to particular printer characteristics.

2.5 Field of View

As mentioned above, the tag 4 is designed to allow all tag data to berecovered from an imaging field of view roughly the size of the tag. Anydata common to a set of contiguous tags only needs to appear once withineach tag, since fragments of the common data can be recovered fromadjacent tags. Any data common only to a column or row of tags mustappear twice within the tag—i.e. once in each horizontal half orvertical half of the tag respectively. And any data unique to the tagmust appear four times within the tag—i.e. once in each quadrant.

Although data which is common to a set of tags, in one or both spatialdimensions, may be decoded from fragments from adjacent tags,pulse-position modulated values are best decoded from spatially-coherentsamples, since this allows raw sample values to be compared withoutfirst being normalised. This implies that the field of view must belarge enough to contain two complete copies of each such pulse-positionmodulated value. The tag is designed so that the maximum extent of apulse-position modulated value is two macrodots. Making the field ofview at least as large as the tag plus two macrodot units guaranteesthat pulse-position modulated values can be coherently sampled.

The only exceptions are the translation codes described in the nextsection, which are three macrodot units long. However, these are highlyredundant and the loss of up to four symbols at the edge of the field ofview is not a problem.

2.6 Encoded Codes and Codewords

In the following section, each symbol in FIGS. 11 to 14 is shown with aunique label. The label consists of an alphabetic prefix whichidentifies which codeword the symbol is part of, and a numeric suffixwhich indicates the index of the symbol within the codeword. Forsimplicity only data symbols 304 are shown, not orientation andtranslation code symbols.

Although some symbol labels are shown rotated to indicate the symmetryof the layout of certain codewords, the layout of each symbol isdetermined by its position within a symbol group and not by the rotationof the symbol label (as described in, for example, the Applicant's USPublication No. 2006/146069).

2.6.1 Orientation Code

The orientation code consists of a single symbol which contains two bitsof data, and is encoded using pulse position modulation. FIG. 8 showsthe layout of the orientation code symbol.

As shown in FIG. 4, the orientation code symbol layout appears oncewithin each symbol group to indicate the orientation of the tag (via theOR symbol).

Each symbol group encodes a one-symbol 4-ary orientation code. The codeis defined by the set of codewords {{0}, {1}, {2}, {3}}. These codewordscorrespond to clockwise tag rotations of 0, 90, 180 and 270 degreesrespectively. Each codeword corresponds to its predecessor read at anorientation of 90 degrees, hence a single codeword gives rise to theentire code when rotated. The code has a minimum distance of 1. Thecodes of an entire tag form a code with a minimum distance of 9,allowing 4 symbol errors to be corrected. If additional symbols arevisible within the field of view then they can be used for additionalredundancy and even more robust decoding. A minimum of three orientationcodes, with a combined minimum distance of 3, must be decoded to allow asingle symbol error to be corrected.

2.6.2 Translation Code

Each translation code symbol encodes one of three values {0, 1, 2}, andis encoded using pulse position modulation i.e. using a single macrodotin one of three positions {p₀, p₁, p₂}. FIG. 8 shows the layout of thetranslation code symbol.

As shown in FIG. 4, the translation code symbol layout appears twice attwo orientations within a symbol group to indicate the horizontal andvertical translation of the tag (via the HT and VT symbolsrespectively).

Each row of symbol groups and each column of symbol groups encodes athree-symbol 3-ary cyclic position code (The Applicant's cyclic positioncodes are described in U.S. Pat. No. 7,082,562, the contents of which isherein incorporated by reference). The code is defined by the codeword{0, 1, 2}. It has a minimum distance of 3, allowing a single symbolerror to be corrected. The codes of an entire tag form a code with aminimum distance of 9, allowing 4 symbol errors to be corrected. Ifadditional symbols are visible within the field of view then they can beused for additional redundancy and even more robust decoding.

The top left corner of an un-rotated tag is identified by a symbol groupwhich encodes the first symbol in two orthogonal cyclic positioncodewords.

2.6.3 Flag Code

The flag symbol consists of one bit of data, and is encoded using 1-bitpulse-position modulation, i.e. using a single macrodot in one of twopositions {p₀, p₁}. FIG. 10 shows the layout of the flag symbol.

The flag symbol is unique to a tag 4 and is therefore coded redundantlyin each quadrant of the tag. As FIG. 10 shows, the flag symbol isreplicated twice but is defined in four ways within each symbol group303. This guarantees that at least four distinct copies of the flagsymbol can be recovered from a quadrant of the tag. Four symbols form acode with a minimum distance of 3, allowing a single error to becorrected. If additional symbols are visible within the field of viewthen they can be used for additional redundancy.

2.6.4 Coordinate Data

The tag contains an x-coordinate codeword and a y-coordinate codewordused to encode the x and y coordinates of the tag respectively. Thecodewords are of a punctured 2⁴-ary (9,4) Reed-Solomon code. The tagtherefore encodes up to 20 bits of information for each coordinate.

Each x coordinate codeword is replicated twice within the tag—in eachhorizontal half (“north” and “south”), and is constant within the columnof tags containing the tag. Likewise, each y coordinate codeword isreplicated twice within the tag—in each vertical half (“east” and“west”), and is constant within the row of tags containing the tag. Thisguarantees that an image of the tag pattern large enough to contain acomplete tag is guaranteed to contain a complete instance of eachcoordinate codeword, irrespective of the alignment of the image with thetag pattern. The instance of either coordinate codeword may consist offragments from different tags.

It should be noted that, in the present invention, some coordinatesymbols are not replicated and are placed on the dividing line betweenthe two halves of the tag. This arrangement saves tag space since thereare not two complete replications of each x-coordinate codeword and eachy-coordinate codeword contained in a tag. Since the field of view is atleast two macrodot units larger than the tag (as discussed in Section2.5), the coordinate symbols placed on the dividing line (having a width2 macrodot units) are still captured when the surface is imaged. Hence,each interaction with the coded surface still provides the tag location.

The layout of the x-coordinate codeword is shown in FIG. 11. The layoutof the y-coordinate codeword is shown in FIG. 12. It can be seen thatx-coordinate symbols X6, X7 and X8 are placed in a central column 310 ofthe tag 4, which divides the eastern half of the tag from the westernhalf. Likewise, the y-coordinate symbols Y6, Y7 and Y8 are placed in acentral row 312 of the tag 4, which divides the northern half of the tagfrom the southern half.

The central column 310 and central row 312 each have a width q, whichcorresponds to the width of one half-symbol i.e. 2s, where s is themacrodot spacing.

Note that a and b suffixes on symbol names indicate low-order andhigh-order symbol halves (h₀ and h₁) respectively.

2.6.5 Common Data

The tag 4 contains three codewords B, C and D which encode informationcommon to a set of contiguous tags in a surface region. Each codeword isof a 2⁴-ary (15,11) Reed-Solomon code. The tag therefore encodes up to132 bits of information common to a set of contiguous tags.

The common codewords are replicated throughout a tagged region. Thisguarantees that an image of the tag pattern large enough to contain acomplete tag is guaranteed to contain a complete instance of each commoncodeword, irrespective of the alignment of the image with the tagpattern. The instance of each common codeword may consist of fragmentsfrom different tags.

The layout of the common codewords is shown in FIG. 13. The codewordshave the same layout, rotated 90 degrees relative to each other.

The tag optionally contains a fourth codeword E with the same (rotated)layout as the common codewords. This codeword is used to encode asecret-key signature or a fragment of an embedded data object. These arediscussed further in Sections 2.6.6 and Section 2.8.3.

2.6.6 Secret-Key Signature

The tag optionally contains an entire secret-key digital signaturecommon to a set of contiguous tags in a surface region. The signatureconsists of fifteen 2⁴-ary symbols. The tag therefore optionally encodesup to 60 bits of secret-key signature data.

The signature is replicated throughout a tagged region. This guaranteesthat an image of the tag pattern large enough to contain a complete tagis guaranteed to contain a complete instance of the signature,irrespective of the alignment of the image with the tag pattern. Theinstance of the signature may consist of fragments from different tags.

The signature, if present, is encoded in the E codeword described inSection 2.6.5.

Digital signatures are discussed further in Section 2.8.4.

2.6.7 Complete Tag

FIG. 14 shows the layout of the data of a complete tag, with each symbolgroup comprising ten data symbols. The orientation and translation codesare not shown in FIG. 14.

2.7 Reed-Solomon Encoding

2.7.1 Reed-Solomon Codes

All data is encoded using a Reed-Solomon code defined over GF(16). Thecode has a natural length n of 15. It is punctured as appropriate toobtain a chosen length. The dimension k of the code is chosen to balancethe error correcting capacity and data capacity of the code, which are(n−k)/2 and k symbols respectively.

The code has the following primitive polynominal:p(x)=x ⁴ +x+1

The code has the following generator polynominal:

${g(x)} = {\prod\limits_{i = 1}^{n - k}\left( {x + \alpha^{t}} \right)}$

For a detailed description of Reed-Solomon codes, refer to Wicker, S. B.and V. K. Bhargava, eds., Reed-Solomon Codes and Their Applications,IEEE Press, 1994.

2.7.2 Codeword Organization

As shown in FIG. 15, redundancy coordinates r_(i) and data coordinatesd_(i) of the code are indexed from left to right according to the powerof their corresponding polynomial terms. The symbols X_(i) of a completecodeword are indexed from right to left to match the bit order of thedata. The bit order within each symbol is the same as the overall bitorder.

2.6.3 Code Instances

Table 1 defines the parameters of the different codes used in the tag.

TABLE 1 Codeword instances error- correcting data length dimensioncapacity capacity name description (n) (k) (symbols) (bits) X, Ycoordinate 9 4 2 20 codewords (see Section 2.6.4) B, C, D, E common 1511 2 44 codewords (see Section 2.6.5)2.7 Tag Coordinate Space

The tag coordinate space has two orthogonal axes labelled x and yrespectively. When the positive x axis points to the right then thepositive y axis points down.

The surface coding does not specify the location of the tag coordinatespace origin on a particular tagged surface, nor the orientation of thetag coordinate space with respect to the surface. This information isapplication-specific. For example, if the tagged surface is a sheet ofpaper, then the application which prints the tags onto the paper mayrecord the actual offset and orientation, and these can be used tonormalise any digital ink subsequently captured in conjunction with thesurface.

The position encoded in a tag is defined in units of tags. Byconvention, the tag position is taken to be the position of the top lefttarget in each tag.

2.8 Tag Information Content

2.8.1 Field Definitions

Table 2 defines the information fields embedded in the surface coding.

TABLE 2 Field Definitions width field (bits) description unique to tagactive area flag 1 A flag indicating whether the area^(a) immediatelysurrounding a tag intersects an active area. b′1′ indicatesintersection. x coordinate 20 The unsigned x coordinate of the tag^(b).y coordinate 20 The unsigned y coordinate of the tag^(b). common totagged region encoding format 4 The format of the encoding. 0: thepresent encoding. Other values are reserved region flags 10 Flagscontrolling the interpretation of region data (see Table 3). coordinateprecision 2 A value (p) indicating the precision of x and y coordinatesaccording to the formula 8 + 4p. macrodot size ID 4 The ID of themacrodot size. 0: the nominal macrodot size^(c). region ID 96 The ID ofthe region containing the tags. secret-key signature 60 A secret-keysignature of the region. CRC (Cyclic Redundancy 16 A CRC^(d) of commontag data. Check) ^(a)the diameter of the area, centered on the tag, isnominally 2.5 times the diagonal size of the tag; this is to accommodatethe worst-case distance between the nib position and the imaged tag^(b)allows a maximum coordinate value of 3.3 km for the nominal tag sizeof 3.14 mm (based on nominal macrodot size and 33 macrodots per tag)^(c)95 microns (based on 1600 dpi and 6 dots per macrodot) ^(d)CCITTCRC-16 [see ITU, Interface between Data Terminal Equipment (DTE) andData Circuit-terminating Equipment (DCE) for terminals operating in thepacket mode and connected to public data networks by dedicated circuit,ITU-T X.25 (10/96)], computed in bit order on raw codeword data (seeTable 4).

An active area is an area within which any captured input should beimmediately forwarded to the corresponding Netpage server 10 forinterpretation. This also allows the Netpage server 10 to signal to theuser that the input has had an immediate effect. Since the server hasaccess to precise region definitions, any active area indication in thesurface coding can be imprecise so long as it is inclusive.

TABLE 3 Region flags bit meaning 0 Region ID is an EPC. Used forHyperlabel (see, for example, U.S. Pat. No. 7,225,979). Otherwise theregion ID is a Netpage region ID. 1 Region ID has a secret-key signature(see Section 2.8.4). 2 Region has embedded data (see Section 2.8.3).Otherwise the region contains no embedded data. 3 Embedded data is apublic-key signature (see Section 2.8.4). Otherwise the data type isspecified in the embedded data block. 4 Embedded public-key signature isshort (see Section 2.8.4). 5 EPC contains a layout number. Used fornon-serialized Hyperlabel applications, where the serial number isreplaced by a layout number (see US2007/0108285). Otherwise the EPCcontains a serial number. 6 Region is non-interactive i.e. x and ycoordinates are zero. Otherwise x and y coordinates are present. 7Region is active i.e. the entire region is an active area and the activearea flag is not present. Otherwise the active area is indicated byindividual tags' active area flags. other Reserved for future use. Mustbe zero.2.8.2 Mapping of Fields to Codewords

Table 4 defines how the information fields map to codewords.

TABLE 3 Mapping of fields to codewords codeword codeword bits fieldwidth field bits X all x coordinate^(a) 20 all Y all y coordinate^(a) 20all F all active area flag 1 all B 27:0  region ID 28 27:0  43:28CRC^(b) 16 all C 3:0 encoding format 4 all 13:4  region flags 10 all15:14 coordinate precision 2 all 19:16 macrodot size ID 4 all 43:20region ID 24 51:28 D all region ID 44 95:52 E all data fragment 44 all EAll^(c) secret-key signature 60 all ^(a)encoded with leading zeros ifcoordinate precision is less than maximum ^(b)the CRC is computed in bitorder on the data portions of the B, C and D codewords, in that order,excluding the CRC field itself ^(c)entire codeword is used for data i.e.there is no redundancy

As shown in Table 4, codeword E either contains a data fragment or asecret-key signature. These are described in Section 2.8.3 and Section2.6.6 respectively. The secret-key signature is present in a particulartag if the “region has secret-key signature” flag in the region flags isset, and the tag's active area flag is set. The data fragment is presentif the “region contains embedded data” flag in the region flags is setand the tag's active area flag is not set.

When the region flags indicate that a particular codeword is absent thenthe codeword is not coded in the tag pattern, i.e. there are nomacrodots representing the codeword. This applies to the X, Y, F and Ecodewords.

2.8.3 Embedded Data Object

If the “region contains embedded data” flag in the region flags is setthen the surface coding contains embedded data. The embedded data isencoded in multiple contiguous tags' data fragments, and is replicatedin the surface coding as many times as it will fit.

The embedded data is encoded in such a way that a random and partialscan of the surface coding containing the embedded data can besufficient to retrieve the entire data. The scanning system reassemblesthe data from retrieved fragments, and reports to the user whensufficient fragments have been retrieved without error.

As shown in Table 5, each block has a data capacity of 176-bits. Theblock data is encoded in the data fragments of a contiguous group offour tags arranged in a 2×2 square. A tag belongs to a block whoseinteger coordinate is the tag's coordinate divided by 2. Within eachblock the data is arranged into tags with increasing x coordinate withinincreasing y coordinate.

The block parameters are as defined in Table 5. The E codeword of eachtag may encode a fragment of the embedded data.

TABLE 5 Block parameters parameter value description w 2 The width ofthe block, in tags h 2 The height of the block, in tags. b 176 The datacapacity of the block, in bits

If the E codeword of a particular tag does not contain a fragment of theembedded data, then the pen 101 can discover this implicitly by thefailure of the codeword to decode, or explicitly from the tag's activearea flag.

Data of arbitrary size may be encoded into a superblock consisting of acontiguous set of blocks arranged in a rectangle. The size of thesuperblock may be encoded in each block. A block belongs to a superblockwhose integer coordinate is the block's coordinate divided by thesuperblock size. Within each superblock the data is arranged into blockswith increasing x coordinate within increasing y coordinate.

The superblock is replicated in the surface coding as many times as itwill fit, including partially along the edges of the surface coding.

The data encoded in the superblock may include more precise typeinformation, more precise size information, and more extensive errordetection and/or correction data.

2.8.4 Digital Signatures

As described in Section 2.8.1, a region may contain a secret-key digitalsignature. In an online environment the secret-key signature can beverified, in conjunction with the region ID, by querying a server withknowledge of the secret-key signature or the corresponding secret key.

If the region contains embedded data and the “embedded data is apublic-key signature” flag in the region flag is set then the surfacecoding contains an embedded public-key digital signature of the regionID.

If the “embedded public-key signature is short” flag is set, then theembedded public-key signature is a 160-bit signature encoded in a singleblock consisting of just the signature and a 16-bit CRC, i.e. with thesuperblock parameters omitted.

In an online environment any number of signature fragments can be used,in conjunction with the region ID and optionally the secret-keysignature, to validate the public-key signature by querying a serverwith knowledge of the full public-key signature or the correspondingprivate key.

In an offline (or online) environment the entire public-key signaturecan be recovered by reading multiple tags, and can then be verifiedusing the corresponding public signature key. The actual length and typeof the signature are determined from the region ID during signaturevalidation.

Digital signature verification is discussed in the Applicant's USPublication No. 2007/0108285, the contents of which are hereinincorporated by reference.

2.9 Tag Imaging and Decoding

The minimum imaging field of view required to guarantee acquisition ofdata from an entire tag has a diameter of 49.5s (i.e. ((3×11)+2)√2s),allowing for arbitrary rotation and translation of the surface coding inthe field of view. Notably, the imaging field of view does not have tobe large enough to guarantee capture of an entire tag—the arrangement ofthe data symbols within each tag ensures that a any square portion oflength (l+2s) captures the requisite information in full, irrespectiveof whether a whole tag is actually visible in the field-of-view. As usedherein, l is defined as the length of a tag.

In terms of imaging the coding pattern, the imaging field-of-view istypically a circle. Accordingly, the imaging field-of-view shouldpreferably have diameter of at least (l+2s)√2 and less than two tagdiameters. Importantly, the field-of-view is not required to be at leasttwo tag diameters, in contrast with prior art tag designs, because it isnot essential in the present invention to capture an entire tag in thefield of view.

The extra two macrodot units ensure that pulse-position modulated valuescan be decoded from spatially coherent samples. Furthermore, the extratwo macrodot units ensure that coordinate symbols from a central columnor row of a tag (see Section 2.6.4) are readable from every interactionwith the surface.

In the present context, a “tag diameter” is given to mean the length ofa tag diagonal.

Given a maximum macrodot spacing of 106 microns (e.g. for 1200 dpi),this gives a required field of view of 5.24 mm.

Table 6 gives pitch ranges achievable for the present surface coding fordifferent sampling rates and hence image sensor array sizes.

TABLE 6 Pitch ranges achievable for present surface coding for differentimage sensor sizes; dot pitch = 600 dpi, macrodot pitch = 2 dots, fieldof view = 4.19 mm, viewing distance = 30 mm, nib-to-FOV separation = 1mm pitch range roll range sampling image sensor scaled^(a) image(degrees) (degrees) rate size sensor size −30 to +38 −34 to +34 2 130163 2.5 163 204 −35 to +44 −39 to +39 2 140 175 2.5 175 219 −40 to +48−44 to +44 2 153 291 2.5 191 239 −45 to +53 −48 to +48 2 168 210 2.5 210263 ^(a)scaled by 1.25 (i.e. 106 microns/85 microns) to accommodatemaximum macrodot spacing

FIG. 16 shows a tag image processing and decoding process flow up to thestage of sampling and decoding the data codewords. Firstly, a raw image802 of the tag pattern is acquired (at 800), for example via an imagesensor such as a CCD image sensor, CMOS image sensor, or a scanninglaser and photodiode image sensor. The raw image 802 is then typicallyenhanced (at 804) to produce an enhanced image 806 with improvedcontrast and more uniform pixel intensities. Image enhancement mayinclude global or local range expansion, equalisation, and the like. Theenhanced image 806 is then typically filtered (at 808) to produce afiltered image 810. Image filtering may consist of low-pass filtering,with the low-pass filter kernel size tuned to obscure macrodots 302 butto preserve targets 301. The filtering step 808 may include additionalfiltering (such as edge detection) to enhance target features 301.Encoding of data codewords 304 using pulse position modulation (PPM)provides a more uniform coding pattern 3 3 than simple binary dotencoding (as described in, for example, U.S. Pat. No. 6,832,717).Advantageously, this helps separate targets 301 from data areas, therebyallowing more effective low-pass filtering of the PPM-encoded datacompared to binary-coded data.

Following low-pass filtering, the filtered image 810 is then processed(at 812) to locate the targets 301. This may consist of a search fortarget features whose spatial inter-relationship is consistent with theknown geometry of the tag pattern. Candidate targets may be identifieddirectly from maxima in the filtered image 810, or may be the subject offurther characterization and matching, such as via their (binary orgrayscale) shape moments (typically computed from pixels in the enhancedimage 806 based on local maxima in the filtered image 810), as describedin U.S. Pat. No. 7,055,739, the contents of which is herein incorporatedby reference.

The identified targets 301 are then assigned to a target grid 816. Eachcell of the grid 816 contains a symbol group 303, and several symbolgroups will of course be visible in the image. At this stage, individualtags 4 will not be identifiable in the target grid 816, since thetargets 301 do not demarcate one tag from another.

To allow macrodot values to be sampled accurately, the perspectivetransform of the captured image must be inferred. Four of the targets301 are taken to be the perspective-distorted corners of a square ofknown size in tag space, and the eight-degree-of-freedom perspectivetransform 822 is inferred (at 820), based on solving the well-understoodequations relating the four tag-space and image-space point pairs.Calculation of the 2D perspective transform is described in detail in,for example, Applicant's U.S. Pat. No. 6,832,717, the contents of whichis herein incorporated by reference.

Since each image will contain at least 9, at least 16 or at least 25targets arranged in a square grid, the accuracy of calculating the 2Dperspective transform is improved compared to the Applicant's previoustag designs described in, for example, U.S. Pat. No. 6,832,717. Hence,more accurate position calculation can be achieved with the tag designof the present invention.

The inferred tag-space to image-space perspective transform 822 is usedto project each known macrodot position in tag space into image space.Since all bits in the tags are represented by PPM-encoding, the presenceor absence of each macrodot 302 can be determined using a localintensity reference, rather than a separate intensity reference. Thus,PPM-encoding provides improved data sampling compared with pure binaryencoding.

The next stage determines the orientation of the tag(s), or portionsthereof, in the field of view. At least 3 orientation codewords aresampled and decoded (at 824) to provide the orientation 826. Robustorientation determination is provided since many symbol groups 303 arecontained in the image, with each symbol group containing an orientationsymbol, as described above. Moreover, and as described in Section 2.5.1,since N orientation symbols in a tag form a code with minimum distanceN, the code is capable of correcting (N−1)/2 errors. Hence, orientationdetermination is very robust and capable of correcting errors, dependingon the number of orientation symbols sampled.

After determination of the orientation 826, the next stage samples anddecodes two or more orthogonal translation codewords (at 828) todetermine the relative translation 830 of tags(s) in the field of viewrelative to the target grid. This enables alignment of the tags 4 withthe target grid 818, thereby allowing individual tag(s), or portionsthereof, to be distinguished in the coding pattern 3 in the field ofview. Since each symbol group 303 contains a translation code, multipletranslation codes can be sampled to provide robust translationdetermination. As described in Section 2.5.2, the translation code is acyclic position code. Since each row and each column of a tag contains Msymbol groups, the code has minimum distance M×M. This allows veryrobust determination of the alignment of tags 4 with the target grid818. The alignment needs to be both robust and accurate since there aremany possible alignments when each tag 4 contains multiple symbol groups303.

Once initial imaging and decoding has yielded the 2D perspectivetransform, the orientation, and the translation of tag(s) relative tothe target grid, the data codewords 304 can then be sampled and decoded836 to yield the requisite decoded codewords 838.

Decoding of the data codewords 304 typically proceeds as follows:

-   -   sample common Reed-Solomon codewords    -   decode common Reed-Solomon codewords    -   verify tag data CRC    -   on decode error flag bad region ID sample    -   determine encoding type, and reject unknown encoding    -   determine region flags    -   determine region ID    -   sample and decode x and y coordinate Reed-Solomon codewords    -   determine tag x-y location from codewords    -   determine nib x-y location from tag x-y location and perspective        transform    -   sample and decode four or more flag symbols to determine active        area flag    -   determine active area status of nib location with reference to        active area flag    -   encode region ID, nib x-y location, and nib active area status        in digital ink (“interaction data”)    -   route digital ink based on region flags

The skilled person will appreciate that the decoding sequence describedabove represents one embodiment of the present invention. It will, ofcourse, be appreciated that the interaction data sent from the pen 101to the netpage system may include other data e.g. digital signature (seeSection 2.8.4), pen mode (see US 2007/125860), orientation data, pen ID,nib ID etc.

An example of interpreting interaction data, received by the netpagesystem from the netpage pen 101, is discussed briefly above. A moredetailed discussion of how the netpage system may interpret interactiondata can be found in the Applicant's previously-filed applications (see,for example, US 2007/130117 and US 2007/108285, the contents of whichare herein incorporated by reference).

3. Netpage Pen

3.1 Functional Overview

The active sensing device of the netpage system may take the form of aclicker (for clicking on a specific position on a surface), a pointerhaving a stylus (for pointing or gesturing on a surface using pointerstrokes), or a pen having a marking nib (for marking a surface with inkwhen pointing, gesturing or writing on the surface). For a descriptionof various netpage sensing devices, reference is made to U.S. Pat. Nos.7,105,753; 7,015,901; 7,091,960; and US Publication No. 2006/0028459,the contents of each of which are herein incorporated by reference.

It will be appreciated that the present invention may utilize anysuitable optical reader. However, the Netpage pen 400 will be describedherein as one such example.

The Netpage pen 400 is a motion-sensing writing instrument which worksin conjunction with a tagged Netpage surface (see Section 2). The penincorporates a conventional ballpoint pen cartridge for marking thesurface, an image sensor and processor for simultaneously capturing theabsolute path of the pen on the surface and identifying the surface, aforce sensor for simultaneously measuring the force exerted on the nib,and a real-time clock for simultaneously measuring the passage of time.

While in contact with a tagged surface, as indicated by the forcesensor, the pen continuously images the surface region adjacent to thenib, and decodes the nearest tag in its field of view to determine boththe identity of the surface, its own instantaneous position on thesurface and the pose of the pen. The pen thus generates a stream oftimestamped position samples relative to a particular surface, andtransmits this stream to the Netpage server 10. The sample streamdescribes a series of strokes, and is conventionally referred to asdigital ink (DInk). Each stroke is delimited by a pen down and a pen upevent, as detected by the force sensor. More generally, any dataresulting from an interaction with a Netpage, and transmitted to theNetpage server 10, is referred to herein as “interaction data”.

The pen samples its position at a sufficiently high rate (nominally 100Hz) to allow a Netpage server to accurately reproduce hand-drawnstrokes, recognise handwritten text, and verify hand-written signatures.

The Netpage pen also supports hover mode in interactive applications. Inhover mode the pen is not in contact with the paper and may be somesmall distance above the surface of the paper (or other substrate). Thisallows the position of the pen, including its height and pose to bereported. In the case of an interactive application the hover modebehaviour can be used to move a cursor without marking the paper, or thedistance of the nib from the coded surface could be used for toolbehaviour control, for example an air brush function.

The pen includes a Bluetooth radio transceiver for transmitting digitalink via a relay device to a Netpage server. When operating offline froma Netpage server the pen buffers captured digital ink in non-volatilememory. When operating online to a Netpage server the pen transmitsdigital ink in real time.

The pen is supplied with a docking cradle or “pod”. The pod contains aBluetooth to USB relay. The pod is connected via a USB cable to acomputer which provides communications support for local applicationsand access to Netpage services.

The pen is powered by a rechargeable battery. The battery is notaccessible to or replaceable by the user. Power to charge the pen can betaken from the USB connection or from an external power adapter throughthe pod. The pen also has a power and USB-compatible data socket toallow it to be externally connected and powered while in use.

The pen cap serves the dual purpose of protecting the nib and theimaging optics when the cap is fitted and signalling the pen to leave apower-preserving state when uncapped.

3.2 Ergonomics and Layout

FIG. 17 shows a rounded triangular profile gives the pen 400 anergonomically comfortable shape to grip and use the pen in the correctfunctional orientation. It is also a practical shape for accommodatingthe internal components. A normal pen-like grip naturally conforms to atriangular shape between thumb 402, index finger 404 and middle finger406.

As shown in FIG. 18, a typical user writes with the pen 400 at a nominalpitch of about 30 degrees from the normal toward the hand 408 when held(positive angle) but seldom operates a pen at more than about 10 degreesof negative pitch (away from the hand). The range of pitch angles overwhich the pen 400 is able to image the pattern on the paper has beenoptimised for this asymmetric usage. The shape of the pen 400 helps toorient the pen correctly in the user's hand 408 and to discourage theuser from using the pen “upside-down”. The pen functions “upside-down”but the allowable tilt angle range is reduced.

The cap 410 is designed to fit over the top end of the pen 400, allowingit to be securely stowed while the pen is in use. Multi colour LEDsilluminate a status window 412 in the top edge (as in the apex of therounded triangular cross section) of the pen 400 near its top end. Thestatus window 412 remains un-obscured when the cap is stowed. Avibration motor is also included in the pen as a haptic feedback system(described in detail below).

As shown in FIG. 19, the grip portion of the pen has a hollow chassismolding 416 enclosed by a base molding 528 to house the othercomponents. The ink cartridge 414 for the ball point nib (not shown)fits naturally into the apex 420 of the triangular cross section,placing it consistently with the user's grip. This in turn providesspace for the main PCB 422 in the centre of the pen and for the battery424 in the base of the pen. By referring to FIG. 20A, it can be seenthat this also naturally places the tag-sensing optics 426 unobtrusivelybelow the nib 418 (with respect to nominal pitch). The nib molding 428of the pen 400 is swept back below the ink cartridge 414 to preventcontact between the nib molding 428 and the paper surface when the penis operated at maximum pitch.

As best shown in FIG. 20B, the imaging field of view 430 emerges througha centrally positioned IR filter/window 432 below the nib 418, and twonear-infrared illumination LEDs 434, 436 emerge from the two bottomcorners of the nib molding 428. Each LED 434, 436 has a correspondingillumination field 438, 440.

As the pen is hand-held, it may be held at an angle that causesreflections from one of the LED's that are detrimental to the imagesensor. By providing more than one LED, the LED causing the offendingreflections can be extinguished.

Specific details of the pen mechanical design can be found in USPublication No. 2006/0028459, the contents of which are hereinincorporated by reference.

3.3 Pen Feedback Indications

FIG. 21 is a longitudinal cross section through the centre-line if thepen 400 (with the cap 410 stowed on the end of the pen). The penincorporates red and green LEDs 444 to indicate several states, usingcolours and intensity modulation. A light pipe 448 on the LEDs 444transmit the signal to the status indicator window 412 in the tubemolding 416. These signal status information to the user includingpower-on, battery level, untransmitted digital ink, network connectionon-line, fault or error with an action, detection of an “active area”flag, detection of an “embedded data” flag, further data sampling torequired to acquire embedded data, acquisition of embedded datacompleted etc.

A vibration motor 446 is used to haptically convey information to theuser for important verification functions during transactions. Thissystem is used for important interactive indications that might bemissed due to inattention to the LED indicators 444 or high levels ofambient light. The haptic system indicates to the user when:

-   -   The pen wakes from standby mode    -   There is an error with an action    -   To acknowledge a transaction        3.4 Pen Optics

The pen incorporates a fixed-focus narrowband infrared imaging system.It utilizes a camera with a short exposure time, small aperture, andbright synchronised illumination to capture sharp images unaffected bydefocus blur or motion blur.

TABLE 6 Optical Specifications Magnification ^(~)0.225 Focal length of6.0 mm lens Viewing distance 30.5 mm Total track length 41.0 mm Aperturediameter 0.8 mm Depth of field _(.) ^(~)/ 6.5 mm Exposure time 200 usWavelength 810 nm Image sensor size 140 × 140 pixels Pixel size 10 umPitch range ^(~)15 to_(.) 45 deg Roll range ^(~)30 to_(.) 30 deg Yawrange 0 to 360 deg Minimum sampling 2.25 pixels per rate macrodotMaximum pen 0.5 m/s velocity ¹Allowing 70 micron blur radius²Illumination and filter ³Pitch, roll and yaw are relative to the axisof the pen

Cross sections showing the pen optics are provided in FIGS. 22A and 22B.An image of the Netpage tags printed on a surface 548 adjacent to thenib 418 is focused by a lens 488 onto the active region of an imagesensor 490. A small aperture 494 ensures the available depth of fieldaccommodates the required pitch and roll ranges of the pen 400.

First and second LEDs 434 and 436 brightly illuminate the surface 549within the field of view 430. The spectral emission peak of the LEDs ismatched to the spectral absorption peak of the infrared ink used toprint Netpage tags to maximise contrast in captured images of tags. Thebrightness of the LEDs is matched to the small aperture size and shortexposure time required to minimise defocus and motion blur.

A longpass IR filter 432 suppresses the response of the image sensor 490to any coloured graphics or text spatially coincident with imaged tagsand any ambient illumination below the cut-off wavelength of the filter432. The transmission of the filter 432 is matched to the spectralabsorption peak of the infrared ink to maximise contrast in capturedimages of tags. The filter also acts as a robust physical window,preventing contaminants from entering the optical assembly 470.

3.5 Pen Imaging System

A ray trace of the optic path is shown in FIG. 23. The image sensor 490is a CMOS image sensor with an active region of 140 pixels squared. Eachpixel is 10 μm squared, with a fill factor of 93%. Turning to FIG. 24,the lens 488 is shown in detail. The dimensions are:

-   -   D=3 mm    -   R1=3.593 mm    -   R2=15.0 mm    -   X=0.8246 mm    -   Y=1.0 mm    -   Z=0.25 mm

This gives a focal length of 6.15 mm and transfers the image from theobject plane (tagged surface 548) to the image plane (image sensor 490)with the correct sampling frequency to successfully decode all imagesover the specified pitch, roll and yaw ranges. The lens 488 is biconvex,with the most curved surface facing the image sensor. The minimumimaging field of view 430 required to guarantee acquisition ofsufficient tag data with each interaction is dependent on the specificcoding pattern. The required field of view for the coding pattern of thepresent invention is described in Section 2.9.

The required paraxial magnification of the optical system is defined bythe minimum spatial sampling frequency of 2.25 pixels per macrodot forthe fully specified tilt range of the pen 400, for the image sensor 490of 10 μm pixels. Typically, the imaging system employs a paraxialmagnification of 0.225, the ratio of the diameter of the inverted imageat the image sensor to the diameter of the field of view at the objectplane, on an image sensor 490 of minimum 128×128 pixels. The imagesensor 490 however is 140×140 pixels, in order to accommodatemanufacturing tolerances. This allows up to +/−120 μm (12 pixels in eachdirection in the plane of the image sensor) of misalignment between theoptical axis and the image sensor axis without losing any of theinformation in the field of view.

The lens 488 is made from Poly-methyl-methacrylate (PMMA), typicallyused for injection moulded optical components. PMMA is scratchresistant, and has a refractive index of 1.49, with 90% transmission at810 nm. The lens is biconvex to assist moulding precision and features amounting surface to precisely mate the lens with the optical barrelmolding 492.

A 0.8 mm diameter aperture 494 is used to provide the depth of fieldrequirements of the design.

The specified tilt range of the pen is 15.0 to 45.0 degree pitch, with aroll range of 30.0 to 30.0 degrees. Tilting the pen through itsspecified range moves the tilted object plane up to 6.3 mm away from thefocal plane. The specified aperture thus provides a corresponding depthof field of /6.5 mm, with an acceptable blur radius at the image sensorof 16 μm.

Due to the geometry of the pen design, the pen operates correctly over apitch range of 33.0 to 45.0 degrees.

Referring to FIG. 25, the optical axis 550 is pitched 0.8 degrees awayfrom the nib axis 552. The optical axis and the nib axis converge towardthe paper surface 548. With the nib axis 552 perpendicular to the paper,the distance A between the edge of the field of view 430 closest to thenib axis and the nib axis itself is 1.2 mm.

The longpass IR filter 432 is made of CR-39, a lightweight thermosetplastic heavily resistant to abrasion and chemicals such as acetone.Because of these properties, the filter also serves as a window. Thefilter is 1.5 mm thick, with a refractive index of 1.50. Each filter maybe easily cut from a large sheet using a CO₂ laser cutter.

3.6 Electronics Design

TABLE 3 Electrical Specifications Processor ARM7 (Atmel AT91FR40162)running at 80 MHz with 256 kB SRAM and 2 MB flash memory Digital inkstorage 5 hours of writing capacity Bluetooth 1.2 Compliance USBCompliance 1.1 Battery standby 12 hours (cap off), >4 weeks (cap on)time Battery writing 4 hours of cursive writing (81% pen down, timeassuming easy offload of digital ink) Battery charging 2 hours timeBattery Life Typically 300 charging cycles or 2 years (whichever occursfirst) to 80% of initial capacity. Battery ~340 mAh at 3.7 V,Lithium-ion Polymer Capacity/Type (LiPo)

FIG. 26 is a block diagram of the pen electronics. The electronicsdesign for the pen is based around five main sections. These are:

-   -   the main ARM7 microprocessor 574,    -   the image sensor and image processor 576,    -   the Bluetooth communications module 578,    -   the power management unit IC (PMU) 580 and    -   the force sensor microprocessor 582.        3.6.1 Microprocessor

The pen uses an Atmel AT91FR40162 microprocessor (see Atmel, AT91 ARMThumb Microcontrollers—AT91FR40162 Preliminary,http://www.keil.com/dd/docs/datashts/atmel/at91fr40162.pdf) running at80 MHz. The AT91FR40162 incorporates an ARM7 microprocessor, 256 kBytesof on-chip single wait state SRAM and 2 MBytes of external flash memoryin a stack chip package.

This microprocessor 574 forms the core of the pen 400. Its dutiesinclude:

-   -   setting up the Jupiter image sensor 584,    -   decoding images of Netpage coding pattern (see Section 2.9),        with assistance from the image processing features of the image        sensor 584, for inclusion in the digital ink stream along with        force sensor data received from the force sensor microprocessor        582,    -   setting up the power management IC (PMU) 580,    -   compressing and sending digital ink via the Bluetooth        communications module 578, and    -   programming the force sensor microprocessor 582.

The ARM7 microprocessor 574 runs from an 80 MHz oscillator. Itcommunicates with the Jupiter image sensor 576 using a UniversalSynchronous Receiver Transmitter (USRT) 586 with a 40 MHz clock. TheARM7 574 communicates with the Bluetooth module 578 using a UniversalAsynchronous Receiver Transmitter (UART) 588 running at 115.2 kbaud.Communications to the PMU 580 and the Force Sensor microprocessor (FSP)582 are performed using a Low Speed Serial bus (LSS) 590. The LSS isimplemented in software and uses two of the microprocessor's generalpurpose IOs.

The ARM7 microprocessor 574 is programmed via its JTAG port.

3.6.2 Image Sensor

The ‘Jupiter’ Image Sensor 584 (see US Publication No. 2005/0024510, thecontents of which are incorporated herein by reference) contains amonochrome sensor array, an analogue to digital converter (ADC), a framestore buffer, a simple image processor and a phase lock loop (PLL). Inthe pen, Jupiter uses the USRT's clock line and its internal PLL togenerate all its clocking requirements. Images captured by the sensorarray are stored in the frame store buffer. These images are decoded bythe ARM7 microprocessor 574 with help from the ‘Callisto’ imageprocessor contained in Jupiter. The Callisto image processor performs,inter alia, low-pass filtering of captured images (see Section 2.9 andUS Publication No. 2005/0024510) before macrodot sampling and decodingby the microprocessor 574.

Jupiter controls the strobing of two infrared LEDs 434 and 436 at thesame time as its image array is exposed. One or other of these twoinfrared LEDs may be turned off while the image array is exposed toprevent specular reflection off the paper that can occur at certainangles.

3.6.3 Bluetooth Communications Module

The pen uses a CSR BlueCore4-External device (see CSR,BlueCore4—External Data Sheet rev c, 6 Sep. 2004) as the Bluetoothcontroller 578. It requires an external 8 Mbit flash memory device 594to hold its program code. The BlueCore4 meets the Bluetooth v1.2specification and is compliant to v0.9 of the Enhanced Data Rate (EDR)specification which allows communication at up to 3 Mbps.

A 2.45 GHz chip antenna 486 is used on the pen for the Bluetoothcommunications.

The BlueCore4 is capable of forming a UART to USB bridge. This is usedto allow USB communications via data/power socket 458 at the top of thepen 456.

Alternatives to Bluetooth include wireless LAN and PAN standards such asIEEE 802.11 (Wi-Fi) (see IEEE, 802.11 Wireless Local Area Networks,http://grouper.ieee.org/groups/802/11/index.html), IEEE 802.15 (seeIEEE, 802.15 Working Group for WPAN,http://grouper.ieee.org/groups/802/15/index.html), ZigBee (see ZigBeeAlliance, http://www.zigbee.org), and WirelessUSB Cypress (seeWirelessUSB LR 2.4-GHz DSSS Radio SoC,http://www.cypress.com/cfuploads/img/products/cywusb6935.pdf), as wellas mobile standards such as GSM (see GSM Association,http://www.gsmworld.com/index.shtml), GPRS/EDGE, GPRS Platform,http://www.gsmworld.com/technology/gprs/index.shtml), CDMA (see CDMADevelopment Group, http://www.cdg.org/, and Qualcomm,http://www.qualcomm.com), and UMTS (see 3rd Generation PartnershipProject (3GPP), http://www.3gpp.org).

3.6.4 Power Management Chip

The pen uses an Austria Microsystems AS3603 PMU 580 (see AustriaMicrosystems, AS3603 Multi-Standard Power Management Unit Data Sheetv2.0). The PMU is used for battery management, voltage generation, powerup reset generation and driving indicator LEDs and the and the vibratormotor.

The PMU 580 communicates with the ARM7 microprocessor 574 via the LSSbus 590.

3.6.5 Force Sensor Subsystem

The force sensor subsystem comprises a custom Hokuriku force sensor 500(based on Hokuriku, HFD-500 Force Sensor,http://www.hdk.co.jp/pdf/eng/e1381AA.pdf), an amplifier and low passfilter 600 implemented using op-amps and a force sensor microprocessor582.

The pen uses a Silicon Laboratories C8051F330 as the force sensormicroprocessor 582 (see Silicon Laboratories, C8051F330/1 MCU DataSheet, rev 1.1). The C8051F330 is an 8051 microprocessor with on chipflash memory, 10 bit ADC and 10 bit DAC. It contains an internal 24.5MHz oscillator and also uses an external 32.768 kHz tuning fork.

The Hokuriku force sensor 500 is a silicon piezoresistive bridge sensor.An op-amp stage 600 amplifies and low pass (anti-alias) filters theforce sensor output. This signal is then sampled by the force sensormicroprocessor 582 at 5 kHz.

Alternatives to piezoresistive force sensing include capacitive andinductive force sensing (see Wacom, “Variable capacity condenser andpointer”, US Patent Application 20010038384, filed 8 Nov. 2001, andWacom, Technology, http://www.wacom-components.com/english/tech.asp).

The force sensor microprocessor 582 performs further (digital) filteringof the force signal and produces the force sensor values for the digitalink stream. A frame sync signal from the Jupiter image sensor 576 isused to trigger the generation of each force sample for the digital inkstream. The temperature is measured via the force sensormicroprocessor's 582 on chip temperature sensor and this is used tocompensate for the temperature dependence of the force sensor andamplifier. The offset of the force signal is dynamically controlled byinput of the microprocessor's DAC output into the amplifier stage 600.

The force sensor microprocessor 582 communicates with the ARM7microprocessor 574 via the LSS bus 590. There are two separate interruptlines from the force sensor microprocessor 582 to the ARM7microprocessor 574. One is used to indicate that a force sensor sampleis ready for reading and the other to indicate that a pen down/up eventhas occurred.

The force sensor microprocessor flash memory is programmed in-circuit bythe ARM7 microprocessor 574.

The force sensor microprocessor 582 also provides the real time clockfunctionality for the pen 400. The RTC function is performed in one ofthe microprocessor's counter timers and runs from the external 32.768kHz tuning fork. As a result, the force sensor microprocessor needs toremain on when the cap 472 is on and the ARM7 574 is powered down. Hencethe force sensor microprocessor 582 uses a low power LDO separate fromthe PMU 580 as its power source. The real time clock functionalityincludes an interrupt which can be programmed to power up the ARM7 574.

The cap switch 602 is monitored by the force sensor microprocessor 582.When the cap assembly 472 is taken off (or there is a real time clockinterrupt), the force sensor microprocessor 582 starts up the ARM7 572by initiating a power on and reset cycle in the PMU 580.

3.7 Pen Software

The Netpage pen software comprises that software running onmicroprocessors in the Netpage pen 400 and Netpage pod.

The pen contains a number of microprocessors, as detailed in Section3.6. The Netpage pen software includes software running on the AtmelARM7 CPU 574 (hereafter CPU), the Force Sensor microprocessor 582, andalso software running in the VM on the CSR BlueCore Bluetooth module 578(hereafter pen BlueCore). Each of these processors has an associatedflash memory which stores the processor specific software, together withsettings and other persistent data. The pen BlueCore 578 also runsfirmware supplied by the module manufacturer, and this firmware is notconsidered a part of the Netpage pen software.

The pod contains a CSR BlueCore Bluetooth module (hereafter podBlueCore). The Netpage pen software also includes software running inthe VM on the pod BlueCore.

As the Netpage pen 400 traverses a Netpage tagged surface 548, a streamof correlated position and force samples are produced. This stream isreferred to as DInk. Note that DInk may include samples with zero force(so called “Hover DInk”) produced when the Netpage pen is in proximityto, but not marking, a Netpage tagged surface.

The CPU component of the Netpage pen software is responsible for DInkcapture, tag image processing and decoding (in conjunction with theJupiter image sensor 576), storage and offload management, hostcommunications, user feedback and software upgrade. It includes anoperating system (RTOS) and relevant hardware drivers. In addition, itprovides a manufacturing and maintenance mode for calibration,configuration or detailed (non-field) fault diagnosis. The Force Sensormicroprocessor 582 component of the Netpage pen software is responsiblefor filtering and preparing force samples for the main CPU. The penBlueCore VM software is responsible for bridging the CPU UART 588interface to USB when the pen is operating in tethered mode. The penBlueCore VM software is not used when the pen is operating in Bluetoothmode.

The pod BlueCore VM software is responsible for sensing when the pod ischarging a pen 400, controlling the pod LEDs appropriately, andcommunicating with the host PC via USB. For a detailed description ofthe software modules, reference is made to US Publication No.2006/0028459, the contents of which are herein incorporated byreference.

The present invention has been described with reference to a preferredembodiment and number of specific alternative embodiments. However, itwill be appreciated by those skilled in the relevant fields that anumber of other embodiments, differing from those specificallydescribed, will also fall within the spirit and scope of the presentinvention. Accordingly, it will be understood that the invention is notintended to be limited to the specific embodiments described in thepresent specification, including documents incorporated bycross-reference as appropriate. The scope of the invention is onlylimited by the attached claims.

1. A substrate having a coding pattern disposed on a surface thereof,said coding pattern comprising a plurality of target elements defining atarget grid, said target grid comprising a plurality of cells, whereinneighboring cells share target elements and a tag is defined by aplurality of contiguous cells, wherein each tag comprises: a pluralityof data symbols, which comprises a plurality of first data symbols; aplurality of data elements, each of said data symbols being representedby a respective set of said data elements; and one or more flags, saidone or more flags indicating either that: said first data symbols definefirst data contained in said tag; or said first data symbols define afragment of second data, said second data being embedded in a block ofsaid tags, and wherein each cell comprises: one or more translationsymbols encoded by a respective set of said data elements, saidtranslation symbols identifying a geometric translation of said cellrelative to a tag containing said cell.
 2. The substrate of claim 1,wherein said first data encodes a secret-key digital signature.
 3. Thesubstrate of claim 1, wherein said second data encodes a public-keydigital signature.
 4. The substrate of claim 1, wherein said first datasymbols are arranged such that any tag-sized portion of said codingpattern is guaranteed to contain said first data irrespective of whethera whole tag is contained in said portion.
 5. The substrate of claim 1,wherein said surface comprises a plurality of blocks.
 6. The substrateof claim 1, wherein said block has a width w of at least 2 tags and aheight h of at least 2 tags.
 7. The substrate of claim 1, wherein saidone or more flags further indicate whether said tag is contained in anactive area of said surface.
 8. The substrate of claim 7, wherein saidactive area is selected from the group comprising: a hyperlink area, aform field area and a button area.
 9. The substrate of claim 1, whereinsaid coding pattern comprises: a plurality of target elements defining atarget grid, said target grid comprising a plurality of cells, whereinneighboring cells share target elements and wherein each tag is definedby a plurality of contiguous cells.
 10. The substrate of claim 9,wherein each tag comprises M² contiguous square cells, wherein M is aninteger having a value of at least
 2. 11. The substrate of claim 9,wherein a portion of data is represented by a macrodot occupying one ofa plurality of possible positions within a cell, each positionrepresenting one of a plurality of possible data values.
 12. Thesubstrate of claim 9, wherein a n-bit portion of data is represented bya macrodot occupying one of 2^(n) possible positions within a cell, eachposition representing one of 2^(n) possible data values, wherein n is aninteger.
 13. The substrate of claim 9, wherein each cell defines asymbol group, each symbol group comprising a plurality of data symbols.14. The substrate of claim 9, wherein each cell comprises an orientationsymbol encoded by at least one data element, said orientation symbolidentifying an orientation of said coding pattern with respect to saidsurface.
 15. The substrate of claim 1, wherein each data symbolcomprises two halves, each half comprising 2 bits of data represented bya macrodot occupying one of 4 possible positions within said half 16.The substrate of claim 1, wherein each tag comprises a plurality ofsecond data symbols defining at least one local codeword contained insaid tag, said at least one local codeword identifying a location of arespective tag.
 17. The substrate of claim 1, wherein each tag comprisesa plurality of third data symbols, said third data symbols defining oneor more common codewords contained in said tag, wherein said one or morecommon codewords are defined as codewords common to a plurality ofcontiguous tags.
 18. The substrate of claim 17, wherein said third datasymbols are arranged such that any tag-sized portion of said codingpattern is guaranteed to contain said one or more common codewordsirrespective of whether a whole tag is contained in said portion. 19.The substrate of claim 17, wherein said one or more common codewordsencode region identity data uniquely identifying a region of saidsurface.